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/compile/Png.h b/tools/aapt2/compile/Png.h
index aff1da3..a820051 100644
--- a/tools/aapt2/compile/Png.h
+++ b/tools/aapt2/compile/Png.h
@@ -56,7 +56,7 @@
  */
 class PngChunkFilter : public io::InputStream {
  public:
-  explicit PngChunkFilter(const StringPiece& data);
+  explicit PngChunkFilter(const android::StringPiece& data);
 
   bool Next(const void** buffer, int* len) override;
   void BackUp(int count) override;
@@ -71,7 +71,7 @@
  private:
   bool ConsumeWindow(const void** buffer, int* len);
 
-  StringPiece data_;
+  android::StringPiece data_;
   size_t window_start_ = 0;
   size_t window_end_ = 0;
   bool error_ = false;