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/util/Files.cpp b/tools/aapt2/util/Files.cpp
index f5e49f1..c9b3811 100644
--- a/tools/aapt2/util/Files.cpp
+++ b/tools/aapt2/util/Files.cpp
@@ -157,7 +157,7 @@
 
 std::string packageToPath(const StringPiece& package) {
     std::string outPath;
-    for (StringPiece part : util::tokenize<char>(package, '.')) {
+    for (StringPiece part : util::tokenize(package, '.')) {
         appendPath(&outPath, part);
     }
     return outPath;
@@ -199,7 +199,7 @@
         return false;
     }
 
-    for (StringPiece line : util::tokenize<char>(contents, ' ')) {
+    for (StringPiece line : util::tokenize(contents, ' ')) {
         line = util::trimWhitespace(line);
         if (!line.empty()) {
             outArgList->push_back(line.toString());