Summary:
Attempt to fix#7919.
Currently, if the app is launched into the background and you read `AppState.currentState` too soon, you will see the value `'active'` instead of `'background'`. This is because the default value of `AppState.currentState` is hardcoded to be `'active'` and it is initialized with the actual value asynchronously.
This attempts to fix the bug by having the `RCTAppState` module provide the initial state as a module constant.
As noted in #7919, it looks like this fix was already tried and reverted with 0fb3d8de83. zjj010104, hedgerwang, nicklockwood -- can you explain why? I would very much like to get this bug fixed. Nobody has followed up on the issue I filed so I decided to just go ahead and make a PR with my best guess at a fix.
**Test plan (required)**
Built a small app as described in the repro steps for #7919 and verified that, when the app is launched into the background, `init currentState: background` is printed. Also verified that `i
Closes https://github.com/facebook/react-native/pull/8058
Differential Revision: D3554619
fbshipit-source-id: 5d950b85e335765552bbd3cf6ed91534062e35a1
68b0ce657e