Add support for keeping status bar visible in immersive mode

This CL adds support in OngoingCallInteractor to keep
the status bar visible if there is an ongoing call
and an app is in full screen mode.
Also, broke up the OngoingCallState code into multiple
functions to improve readability.

Testing: Manually tested and added new tests.
Bug: b/372657935
Flag: com.android.systemui.status_bar_chips_modernization

Change-Id: I4ca9fa1f90da31d726c4dc76246c91eb48895422