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/xml/XmlUtil.h b/tools/aapt2/xml/XmlUtil.h
index 5365401..1650ac2 100644
--- a/tools/aapt2/xml/XmlUtil.h
+++ b/tools/aapt2/xml/XmlUtil.h
@@ -74,7 +74,7 @@
  *
  * http://schemas.android.com/apk/prv/res/<package>
  */
-std::string BuildPackageNamespace(const StringPiece& package,
+std::string BuildPackageNamespace(const android::StringPiece& package,
                                   bool private_reference = false);
 
 /**
@@ -90,7 +90,7 @@
    * package declaration.
    */
   virtual Maybe<ExtractedPackage> TransformPackageAlias(
-      const StringPiece& alias, const StringPiece& local_package) const = 0;
+      const android::StringPiece& alias, const android::StringPiece& local_package) const = 0;
 };
 
 /**
@@ -100,8 +100,7 @@
  * the namespace of the package declaration was private.
  */
 void TransformReferenceFromNamespace(IPackageDeclStack* decl_stack,
-                                     const StringPiece& local_package,
-                                     Reference* in_ref);
+                                     const android::StringPiece& local_package, Reference* in_ref);
 
 }  // namespace xml
 }  // namespace aapt