fix($rootScope.$on) check listener existense while deregistering

[dmytroyarmak]

Check that listener is still present in $$listeners before decrease
$$listenerCount. It fixes problem with incorrect $$listenerCount after
call deregistering function multiple times.

Closes #9666

[petebacondarwin]

LGTM

[mary-poppins]

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[dmytroyarmak]

Sorry, now it should be signed.

[mary-poppins]

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