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

tools/aapt2/compile/NinePatch.cpp

diff --git a/tools/aapt2/compile/NinePatch.cpp b/tools/aapt2/compile/NinePatch.cpp
index eab5c97..c931da4 100644
--- a/tools/aapt2/compile/NinePatch.cpp
+++ b/tools/aapt2/compile/NinePatch.cpp
@@ -21,10 +21,12 @@
 #include <vector>
 
 #include "androidfw/ResourceTypes.h"
+#include "androidfw/StringPiece.h"
 
-#include "util/StringPiece.h"
 #include "util/Util.h"
 
+using android::StringPiece;
+
 namespace aapt {
 
 // Colors in the format 0xAARRGGBB (the way 9-patch expects it).