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/io/FileSystem.h b/tools/aapt2/io/FileSystem.h
index 84f851f..dfd3717 100644
--- a/tools/aapt2/io/FileSystem.h
+++ b/tools/aapt2/io/FileSystem.h
@@ -59,8 +59,8 @@
   /**
    * Adds a file located at path. Returns the IFile representation of that file.
    */
-  IFile* InsertFile(const StringPiece& path);
-  IFile* FindFile(const StringPiece& path) override;
+  IFile* InsertFile(const android::StringPiece& path);
+  IFile* FindFile(const android::StringPiece& path) override;
   std::unique_ptr<IFileCollectionIterator> Iterator() override;
 
  private: