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.h b/tools/aapt2/ConfigDescription.h
index 97d0f38..65c9617 100644
--- a/tools/aapt2/ConfigDescription.h
+++ b/tools/aapt2/ConfigDescription.h
@@ -20,8 +20,7 @@
 #include <ostream>
 
 #include "androidfw/ResourceTypes.h"
-
-#include "util/StringPiece.h"
+#include "androidfw/StringPiece.h"
 
 namespace aapt {
 
@@ -42,7 +41,7 @@
    * The resulting configuration has the appropriate sdkVersion defined
    * for backwards compatibility.
    */
-  static bool Parse(const StringPiece& str, ConfigDescription* out = nullptr);
+  static bool Parse(const android::StringPiece& str, ConfigDescription* out = nullptr);
 
   /**
    * If the configuration uses an axis that was added after