[CS] 2/ Remove ongoing call flags; make controller a CoreStartable.

Fixes: 254512753
Fixes: 254512681
Fixes: 254512734
Test: start ongoing call then enter immersive mode in an app -> verify
status bar still displays
Test: start ongoing call, then enter immersive mode, then swipe up to
dismiss status bar -> verify status bar disappears
Test: smoke test of the ongoing call chip
Test: atest OngoingCallControllerTest

Change-Id: Id1a75d4c553b16937a1d8c7ede352ac1fb3f7b1b