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/ResourceTable.cpp b/tools/aapt2/ResourceTable.cpp
index 4e6a50a..dd78750 100644
--- a/tools/aapt2/ResourceTable.cpp
+++ b/tools/aapt2/ResourceTable.cpp
@@ -28,6 +28,8 @@
 #include <string>
 #include <tuple>
 
+using android::StringPiece;
+
 namespace aapt {
 
 static bool less_than_type(const std::unique_ptr<ResourceTableType>& lhs,
@@ -87,7 +89,7 @@
 
   std::unique_ptr<ResourceTablePackage> new_package =
       util::make_unique<ResourceTablePackage>();
-  new_package->name = name.ToString();
+  new_package->name = name.to_string();
   return packages.emplace(iter, std::move(new_package))->get();
 }