InCallActivity: consume back key on incoming call screen

Users will not be able to anything if the physical back key is
pressed on the incoming call screen (return to home screen with
nothing responding.)

Forward porting the key consumption from 4.3

Change-Id: Iea0734852bac7ca5a25e19ec9d9ee7c80dba6741
[3/3] InCallUI: allow disabling non intrusive incall ui

Change-Id: I86bd4b46f031004fa90f1d7936263f9dabcd3860