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/ConfigDescription_test.cpp b/tools/aapt2/ConfigDescription_test.cpp
index 1d22ce0..7933568 100644
--- a/tools/aapt2/ConfigDescription_test.cpp
+++ b/tools/aapt2/ConfigDescription_test.cpp
@@ -18,9 +18,12 @@
 
 #include <string>
 
+#include "androidfw/StringPiece.h"
+
 #include "SdkConstants.h"
 #include "test/Test.h"
-#include "util/StringPiece.h"
+
+using android::StringPiece;
 
 namespace aapt {