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/Locale.h b/tools/aapt2/Locale.h
index fc6c448..3d73b2e 100644
--- a/tools/aapt2/Locale.h
+++ b/tools/aapt2/Locale.h
@@ -21,8 +21,7 @@
 #include <vector>
 
 #include "androidfw/ResourceTypes.h"
-
-#include "util/StringPiece.h"
+#include "androidfw/StringPiece.h"
 
 namespace aapt {
 
@@ -40,7 +39,7 @@
   /**
    * Initialize this LocaleValue from a config string.
    */
-  bool InitFromFilterString(const StringPiece& config);
+  bool InitFromFilterString(const android::StringPiece& config);
 
   /**
    * Initialize this LocaleValue from parts of a vector.