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.cpp b/tools/aapt2/compile/Png.cpp
index 7ab05b5..5e15c88 100644
--- a/tools/aapt2/compile/Png.cpp
+++ b/tools/aapt2/compile/Png.cpp
@@ -15,18 +15,21 @@
  */
 
 #include "Png.h"
-#include "Source.h"
-#include "util/BigBuffer.h"
-#include "util/Util.h"
 
-#include <androidfw/ResourceTypes.h>
 #include <png.h>
 #include <zlib.h>
+
 #include <iostream>
 #include <sstream>
 #include <string>
 #include <vector>
 
+#include "androidfw/ResourceTypes.h"
+
+#include "Source.h"
+#include "util/BigBuffer.h"
+#include "util/Util.h"
+
 namespace aapt {
 
 constexpr bool kDebug = false;