AAPT2: Remove usage of u16string

For legacy reasons, we kept around the use of UTF-16 internally
in AAPT2. We don't need this and this CL removes all instances of
std::u16string and StringPiece16. The only places still needed
are when interacting with the ResTable APIs that only operate in
UTF16.

Change-Id: I492475b84bb9014fa13bf992cff447ee7a5fe588
diff --git a/tools/aapt2/Resource.cpp b/tools/aapt2/Resource.cpp
index 9328b69..b7a091e 100644
--- a/tools/aapt2/Resource.cpp
+++ b/tools/aapt2/Resource.cpp
@@ -22,62 +22,62 @@
 
 namespace aapt {
 
-StringPiece16 toString(ResourceType type) {
+StringPiece toString(ResourceType type) {
     switch (type) {
-        case ResourceType::kAnim:          return u"anim";
-        case ResourceType::kAnimator:      return u"animator";
-        case ResourceType::kArray:         return u"array";
-        case ResourceType::kAttr:          return u"attr";
-        case ResourceType::kAttrPrivate:   return u"^attr-private";
-        case ResourceType::kBool:          return u"bool";
-        case ResourceType::kColor:         return u"color";
-        case ResourceType::kDimen:         return u"dimen";
-        case ResourceType::kDrawable:      return u"drawable";
-        case ResourceType::kFraction:      return u"fraction";
-        case ResourceType::kId:            return u"id";
-        case ResourceType::kInteger:       return u"integer";
-        case ResourceType::kInterpolator:  return u"interpolator";
-        case ResourceType::kLayout:        return u"layout";
-        case ResourceType::kMenu:          return u"menu";
-        case ResourceType::kMipmap:        return u"mipmap";
-        case ResourceType::kPlurals:       return u"plurals";
-        case ResourceType::kRaw:           return u"raw";
-        case ResourceType::kString:        return u"string";
-        case ResourceType::kStyle:         return u"style";
-        case ResourceType::kStyleable:     return u"styleable";
-        case ResourceType::kTransition:    return u"transition";
-        case ResourceType::kXml:           return u"xml";
+        case ResourceType::kAnim:          return "anim";
+        case ResourceType::kAnimator:      return "animator";
+        case ResourceType::kArray:         return "array";
+        case ResourceType::kAttr:          return "attr";
+        case ResourceType::kAttrPrivate:   return "^attr-private";
+        case ResourceType::kBool:          return "bool";
+        case ResourceType::kColor:         return "color";
+        case ResourceType::kDimen:         return "dimen";
+        case ResourceType::kDrawable:      return "drawable";
+        case ResourceType::kFraction:      return "fraction";
+        case ResourceType::kId:            return "id";
+        case ResourceType::kInteger:       return "integer";
+        case ResourceType::kInterpolator:  return "interpolator";
+        case ResourceType::kLayout:        return "layout";
+        case ResourceType::kMenu:          return "menu";
+        case ResourceType::kMipmap:        return "mipmap";
+        case ResourceType::kPlurals:       return "plurals";
+        case ResourceType::kRaw:           return "raw";
+        case ResourceType::kString:        return "string";
+        case ResourceType::kStyle:         return "style";
+        case ResourceType::kStyleable:     return "styleable";
+        case ResourceType::kTransition:    return "transition";
+        case ResourceType::kXml:           return "xml";
     }
     return {};
 }
 
-static const std::map<StringPiece16, ResourceType> sResourceTypeMap {
-        { u"anim", ResourceType::kAnim },
-        { u"animator", ResourceType::kAnimator },
-        { u"array", ResourceType::kArray },
-        { u"attr", ResourceType::kAttr },
-        { u"^attr-private", ResourceType::kAttrPrivate },
-        { u"bool", ResourceType::kBool },
-        { u"color", ResourceType::kColor },
-        { u"dimen", ResourceType::kDimen },
-        { u"drawable", ResourceType::kDrawable },
-        { u"fraction", ResourceType::kFraction },
-        { u"id", ResourceType::kId },
-        { u"integer", ResourceType::kInteger },
-        { u"interpolator", ResourceType::kInterpolator },
-        { u"layout", ResourceType::kLayout },
-        { u"menu", ResourceType::kMenu },
-        { u"mipmap", ResourceType::kMipmap },
-        { u"plurals", ResourceType::kPlurals },
-        { u"raw", ResourceType::kRaw },
-        { u"string", ResourceType::kString },
-        { u"style", ResourceType::kStyle },
-        { u"styleable", ResourceType::kStyleable },
-        { u"transition", ResourceType::kTransition },
-        { u"xml", ResourceType::kXml },
+static const std::map<StringPiece, ResourceType> sResourceTypeMap {
+        { "anim", ResourceType::kAnim },
+        { "animator", ResourceType::kAnimator },
+        { "array", ResourceType::kArray },
+        { "attr", ResourceType::kAttr },
+        { "^attr-private", ResourceType::kAttrPrivate },
+        { "bool", ResourceType::kBool },
+        { "color", ResourceType::kColor },
+        { "dimen", ResourceType::kDimen },
+        { "drawable", ResourceType::kDrawable },
+        { "fraction", ResourceType::kFraction },
+        { "id", ResourceType::kId },
+        { "integer", ResourceType::kInteger },
+        { "interpolator", ResourceType::kInterpolator },
+        { "layout", ResourceType::kLayout },
+        { "menu", ResourceType::kMenu },
+        { "mipmap", ResourceType::kMipmap },
+        { "plurals", ResourceType::kPlurals },
+        { "raw", ResourceType::kRaw },
+        { "string", ResourceType::kString },
+        { "style", ResourceType::kStyle },
+        { "styleable", ResourceType::kStyleable },
+        { "transition", ResourceType::kTransition },
+        { "xml", ResourceType::kXml },
 };
 
-const ResourceType* parseResourceType(const StringPiece16& str) {
+const ResourceType* parseResourceType(const StringPiece& str) {
     auto iter = sResourceTypeMap.find(str);
     if (iter == std::end(sResourceTypeMap)) {
         return nullptr;