Telephony: Incoming Call in Background (2/3)(Do Not Merge)

Testing for now

Before:https://app.box.com/s/0ndc00yby1jkf1yson2d
After:https://app.box.com/s/t230s7dl7uu02syg8q4p
      https://app.box.com/s/rjbie5exn285l1vt1z7i

To test this needs to be reverted : http://review.gummyrom.com/#/c/1169/

Allows user to set that Incoming Call UI will remain in background
if screen is on. Instead, a notification is added to StatusBar, which
can be used to bring up the Incoming Call UI. Ringtone and vibration
still occur as per system settings.

This has the same effect as apps marked as "immersive". It prevents
the incoming call from backgrounding the current activity.

Change-Id: I159bd817ebcc39a0733f9433feb29b310adbc2aa