Move FlutterMain.lookupKeyForAsset() to a non-static place for plugins.

[matthew-carroll]

Move FlutterMain.lookupKeyForAsset() to a non-static place for plugins.

We don't want this to be static because it's something that will likely need to be controlled in tests.

[matthew-carroll]

Done. A FlutterAssets object is now exposed from the FlutterPluginBinding and exposes asset path retrieval methods.