Android HandlerThreadScheduler fix (closes #1241)

Some events are not delivered due to race condition around the check to `innerSubscription.isUnsubscribed()` in the posted runnable. It's currently possible for the Handler thread to be delayed enough that innerSubscription becomes unsubscribed, and causing the action not to run.

After cross-checking the code with `NewThreadScheduler`, it seems that the expected behavior is to continue to run the action even if the inner/parent subscription is unsubscribed, and only stop when the outer subscription is unsubscribed (which is already happening with `handler.removeCallbacks(runnable)`).

This fixes #1241.