InCallUI: add Incoming Call Background (Intrusive InCall improvement) (1/2)

last week, as we know we have bug with Intrusive Incall on Back pressed
and we already fix that by disabled Back pressed while Intrusive InCall showing

instead disabled Back pressed, this patch introduce Intrusive Incall will go to background mode
using Notification on Statusbar

Based on SlimRoms patch
https://gerrit.slimroms.net/1857

with changes to follow Intrusive Incall

Change-Id: Id9456a48fc767a7a3fe46912c7cba0d3dee93991