Summary:
This is to fix the issue that if `Animated.parallel`'s callback - `_onTransitionEnd` being triggered twice in a really short period(say quickly double-click the Android's hardware back button), it might try to `setState` at unmounted stage, hence cause app crash.
This will make sure `_onTransitionEnd` only fired after mounted.
Closes https://github.com/facebook/react-native/pull/10878
Differential Revision: D4167266
Pulled By: ericvicenti
fbshipit-source-id: 7361e0ea4e8481b2da3fa39f78cdc0461693631f