Phone: Incoming Call in Background

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.

PS 2: minor fix

Change-Id: Ib98dd74268826d511ce43ac75dd9dd66f774c9e7
Merge remote-tracking branch 'slim/jb4.2'; commit 'HEAD' into HEAD