Do not share MethodChannel across multiple FlutterEngine

[DIG-]

Some flutter plugins can start a background thing that requires a FlutterEngine (like firebase messaging). This create more than one FlutterEngine running at same time.
The current Android implementation shares MethodChannel across any FlutterEngine, by the singleton ChannelManager.
This creates a problem where the last FlutterEngine started will receive all calls from native side, intead of the caller engine..

[DIG-]

This is a possible patch that converts the ChannelManager from a singleton to a class that is initialized in onAttachedToEngine
(DIG-@00e2adc)

patch.patch