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/compile/IdAssigner.h b/tools/aapt2/compile/IdAssigner.h
index 371ec01..9640eb8 100644
--- a/tools/aapt2/compile/IdAssigner.h
+++ b/tools/aapt2/compile/IdAssigner.h
@@ -19,11 +19,11 @@
 
 #include <unordered_map>
 
+#include "android-base/macros.h"
+
 #include "Resource.h"
 #include "process/IResourceTableConsumer.h"
 
-#include "android-base/macros.h"
-
 namespace aapt {
 
 /**
@@ -40,8 +40,7 @@
   bool Consume(IAaptContext* context, ResourceTable* table) override;
 
  private:
-  const std::unordered_map<ResourceName, ResourceId>* assigned_id_map_ =
-      nullptr;
+  const std::unordered_map<ResourceName, ResourceId>* assigned_id_map_ = nullptr;
 };
 
 }  // namespace aapt