FCM should be defined by an interface

[dgaedcke]

We have to wrap FCM in order to mock the static methods for TDD

But I assume static is required for message handling when instances have been garbage collected (e.g. backgrounded)

if would be ideal if FCM respected an interface. We could make our mock respect the same interface and then swapping out the wrapped class for the real FCM would be easier

Do I have this right??