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/File.h b/tools/aapt2/io/File.h
index 644f59f..3d5b5b1 100644
--- a/tools/aapt2/io/File.h
+++ b/tools/aapt2/io/File.h
@@ -110,7 +110,7 @@
  public:
   virtual ~IFileCollection() = default;
 
-  virtual IFile* FindFile(const StringPiece& path) = 0;
+  virtual IFile* FindFile(const android::StringPiece& path) = 0;
   virtual std::unique_ptr<IFileCollectionIterator> Iterator() = 0;
 };