InCallUI: add Incoming Call Background (non-Intrusive InCall improvement)

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

instead disabled Back pressed, this patch introduce non-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