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/ResourceValues.h b/tools/aapt2/ResourceValues.h
index aa1b550..8ae71ad 100644
--- a/tools/aapt2/ResourceValues.h
+++ b/tools/aapt2/ResourceValues.h
@@ -84,15 +84,15 @@
     /**
      * Returns the comment that was associated with this resource.
      */
-    StringPiece16 getComment() const {
+    const std::string& getComment() const {
         return mComment;
     }
 
-    void setComment(const StringPiece16& str) {
+    void setComment(const StringPiece& str) {
         mComment = str.toString();
     }
 
-    void setComment(std::u16string&& str) {
+    void setComment(std::string&& str) {
         mComment = std::move(str);
     }
 
@@ -115,7 +115,7 @@
 
 protected:
     Source mSource;
-    std::u16string mComment;
+    std::string mComment;
     bool mWeak = false;
     bool mTranslateable = true;
 };