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/link/ReferenceLinker.cpp b/tools/aapt2/link/ReferenceLinker.cpp
index 66eb0df..fe886de 100644
--- a/tools/aapt2/link/ReferenceLinker.cpp
+++ b/tools/aapt2/link/ReferenceLinker.cpp
@@ -288,7 +288,7 @@
 
 struct EmptyDeclStack : public xml::IPackageDeclStack {
     Maybe<xml::ExtractedPackage> transformPackageAlias(
-            const StringPiece16& alias, const StringPiece16& localPackage) const override {
+            const StringPiece& alias, const StringPiece& localPackage) const override {
         if (alias.empty()) {
             return xml::ExtractedPackage{ localPackage.toString(), true /* private */ };
         }