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/link/ReferenceLinker.cpp b/tools/aapt2/link/ReferenceLinker.cpp
index be787b2..ea68b61 100644
--- a/tools/aapt2/link/ReferenceLinker.cpp
+++ b/tools/aapt2/link/ReferenceLinker.cpp
@@ -30,6 +30,8 @@
 #include "util/Util.h"
 #include "xml/XmlUtil.h"
 
+using android::StringPiece;
+
 namespace aapt {
 
 namespace {
@@ -192,8 +194,7 @@
       const StringPiece& alias,
       const StringPiece& local_package) const override {
     if (alias.empty()) {
-      return xml::ExtractedPackage{local_package.ToString(),
-                                   true /* private */};
+      return xml::ExtractedPackage{local_package.to_string(), true /* private */};
     }
     return {};
   }