Summary:
## Motivation
The concept behind JSCallInvoker doesn't necessarily have to apply only to the JS thread. On Android, we need to re-use this abstraction to allow execution of async method calls on the NativeModules thread.
Reviewed By: PeteTheHeat
Differential Revision: D17377313
fbshipit-source-id: 3d9075cbfce0b908d800a366947cfd16a3013d1c