Background Restoration

[jacobrosenthal]

When we last left our heroes, the big missing functionality was being awoken from sleep and restoring bluetooth. From my react native issue: facebook/react-native#1660

Right now when an app is launched, the launchoptions are given to a native module via the setBridge function. This is handy for push notifications as you can know what notification launched you, etc.

That paradigm doesnt work for Corelocation and Corebluetooth restoration though. These trigger a didFinishLaunchingWithOptions in order to tell the app that a location or device it was told to track has resurfaced. The rest of the app is never launched though, so setbridge never happens. Its common to do a push notification right there or restart your bluetooth stack to do some action. So I think we need a way to register functions to be called upon those launchoptions keys.

I saw this while skimming code around this issue, could we do this:
https://github.com/facebook/react-native/blob/d5209a082976d272e6e897c41fb6f98feda84081/Examples/UIExplorer/UIExplorer/AppDelegate.m#L29

[jacobrosenthal]

Backgrounding seems to be working now? I need better methods of simulating memory pressure as I so far have been unable to kill the app due to memory pressure to see how we handle being rewoken. We also need to follow the CoreBluetooth Guide in checking to see if weve left services or characteristics undiscovered because of being backgrounded..