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/ResourceParser.h b/tools/aapt2/ResourceParser.h
index ee5b337..ece3090 100644
--- a/tools/aapt2/ResourceParser.h
+++ b/tools/aapt2/ResourceParser.h
@@ -63,7 +63,7 @@
      * contains the escaped and whitespace trimmed text, while `outRawString`
      * contains the unescaped text. Returns true on success.
      */
-    bool flattenXmlSubtree(xml::XmlPullParser* parser, std::u16string* outRawString,
+    bool flattenXmlSubtree(xml::XmlPullParser* parser, std::string* outRawString,
                            StyleString* outStyleString);
 
     /*
@@ -89,7 +89,7 @@
     bool parseAttr(xml::XmlPullParser* parser, ParsedResource* outResource);
     bool parseAttrImpl(xml::XmlPullParser* parser, ParsedResource* outResource, bool weak);
     Maybe<Attribute::Symbol> parseEnumOrFlagItem(xml::XmlPullParser* parser,
-                                                 const StringPiece16& tag);
+                                                 const StringPiece& tag);
     bool parseStyle(xml::XmlPullParser* parser, ParsedResource* outResource);
     bool parseStyleItem(xml::XmlPullParser* parser, Style* style);
     bool parseDeclareStyleable(xml::XmlPullParser* parser, ParsedResource* outResource);