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/diff/Diff.cpp b/tools/aapt2/diff/Diff.cpp
index 1ff6ef6..9b1f057 100644
--- a/tools/aapt2/diff/Diff.cpp
+++ b/tools/aapt2/diff/Diff.cpp
@@ -28,7 +28,7 @@
 
 class DiffContext : public IAaptContext {
 public:
-    const std::u16string& getCompilationPackage() override {
+    const std::string& getCompilationPackage() override {
         return mEmpty;
     }
 
@@ -57,7 +57,7 @@
     }
 
 private:
-    std::u16string mEmpty;
+    std::string mEmpty;
     StdErrDiagnostics mDiagnostics;
     NameMangler mNameMangler = NameMangler(NameManglerPolicy{});
     SymbolTable mSymbolTable;