Move StringPiece to libandroidfw

libandroidfw needs to make use of StringPiece, so
move it to libandroidfw and update all code referencing
StringPiece in aapt2.

Test: make libandroidfw_tests libaapt2_tests
Change-Id: I68d7f0fc7c651b048d9d1f5e7971f10ef5349fa1
diff --git a/tools/aapt2/ResourceValues.h b/tools/aapt2/ResourceValues.h
index ea73615..d380f8d 100644
--- a/tools/aapt2/ResourceValues.h
+++ b/tools/aapt2/ResourceValues.h
@@ -22,6 +22,7 @@
 #include <vector>
 
 #include "androidfw/ResourceTypes.h"
+#include "androidfw/StringPiece.h"
 
 #include "Diagnostics.h"
 #include "Resource.h"
@@ -73,7 +74,7 @@
    */
   const std::string& GetComment() const { return comment_; }
 
-  void SetComment(const StringPiece& str) { comment_ = str.ToString(); }
+  void SetComment(const android::StringPiece& str) { comment_ = str.to_string(); }
 
   void SetComment(std::string&& str) { comment_ = std::move(str); }