Summary:
`BackAndroid` JS event subscriptions should be called in reverse order
(the subscription from the latest `addEventLister` should run first).
Also if listener returns true, don't call other listeners.
**Motivation**: We use `BackAndroid` listeners to prevent closing screens with user's input.
When we have two screens in stack (each screen with listener, which show alerts),
we want to show alert only from the last screen listener, not from all.
Closes https://github.com/facebook/react-native/pull/8929
Differential Revision: D3598978
Pulled By: dmmiller
fbshipit-source-id: a7b0762b36a60755a844e90fffd58887f89c9ffb
b8576312ca