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/compile/Png.cpp b/tools/aapt2/compile/Png.cpp
index bbf7f41..055d8b5 100644
--- a/tools/aapt2/compile/Png.cpp
+++ b/tools/aapt2/compile/Png.cpp
@@ -1234,7 +1234,7 @@
         goto bail;
     }
 
-    if (util::stringEndsWith<char>(source.path, ".9.png")) {
+    if (util::stringEndsWith(source.path, ".9.png")) {
         std::string errorMsg;
         if (!do9Patch(&pngInfo, &errorMsg)) {
             mDiag->error(DiagMessage() << errorMsg);