Separate activation broadcast receiver from channel handler class

[ikbalkaya]

This PR fixes activation handling code by

• Fixes some issues with PushActivationHandlers singleton, by providing an instance method and converting some static methods into instance methods
• Moves activation broadcast receiver to a separate file, creating methods for registering with callback and unregistering.
• Respects the FlutterEngine lifecycle by registering the receiver when it's attached to engine and deregisters it when it's detached.

Closes #304

Please note that changes here won't cause a manual application launch, meaning that any message from broadcast receiver will be received only when the Flutter engine is attached. I did this to prevent further bugs raising from inconsistencies related to Flutter engine lifecycle.
However this is still a temporary solution. A full solution would be to separate third party push plugin so that we can reuse code directly from Firebase library (#292) or completely refactor push notification code on our plugin

Note: Java library will send activation results using broadcasts. That method might have been chosen for native Android apps, but it could be nice if we could have a async alternative that would give results through callback. If so we wouldn't need to deal with broadcast receivers on our plugin.

[KacperKluka]

Overall looks good 👍 One small problem but it was present before so I'll let you decide what to do with it 😉

[ikurek]

LGTM 🙂