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.cpp b/tools/aapt2/ConfigDescription.cpp
index b1bd401..c97d6d4 100644
--- a/tools/aapt2/ConfigDescription.cpp
+++ b/tools/aapt2/ConfigDescription.cpp
@@ -20,15 +20,16 @@
 #include <vector>
 
 #include "androidfw/ResourceTypes.h"
+#include "androidfw/StringPiece.h"
 
 #include "Locale.h"
 #include "SdkConstants.h"
-#include "util/StringPiece.h"
 #include "util/Util.h"
 
-namespace aapt {
-
 using android::ResTable_config;
+using android::StringPiece;
+
+namespace aapt {
 
 static const char* kWildcardName = "any";