Manual merge of Idf6bb25438336616e6453e7afe0e79f227af98d3

Fix 2575838: Update emergency button in AccountUnlockScreen DO NOT MERGE

AccountUnlockScreen wasn't registered to receive updates from
UpdateMonitor. It now correctly updates the Emergency call button
to allow the user to return to the call in progress (InCallScreen).

Tested on WVGA device with keyboard:
- hitting Home during call, observing state, and pressing button to return to call.
- hitting Home while not in call to ensure it still invokes EmergencyDialer.
- above in various keyboard states.

Change-Id: Ic4480ff69ff4479557ddb7f76c751b005b1c60bf