Fix outgoing native calls on iOS when app is killed

[manuquentin]

On iOS when waking up the app with a native outgoing call, we have sometimes an error :

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Error when sending event: RNCallKeepDidReceiveStartCallAction with body: {
    handle = "\U202d802-6\U202c";
    video = 0;
}. Bridge is not set. This is probably because you've explicitly synthesized the bridge in RNCallKeep, even though it's inherited from RCTEventEmitter.'
*** First throw call stack:
(0x19366127c 0x19283b9f8 0x19357a988 0x19408aee8 0x10046c170 0x100747014 0x1930a17d4 0x193046018 0x193055fa4 0x19304eee8 0x1935f2c1c 0x1935edb54 0x1935ed0b0 0x1957ed79c 0x1bfe1c978 0x1002dbe8c 0x1930b28e0)

This is due to the observer that doesn't share the same instance of CallKeep. I've fixed that with a Singleton.

Another issue we have is this error :

Sending `RNCallKeepHandleStartCallNotification` with no listeners registered.

This is due to the app that didn't bind the event before it is sent. Event when we bind this event directly at the top of index.ios.js.

I've increased the workaround delay. I don't have any other idea to fix that...