Fix the issue that automatic mute state remains ON after adding VT call

An issue occurs because InCallFragment#onResume() that release the
automatic mute state is not called when switching fragments by adding
call.

To resolve this issue, handle automatic mute state in InCallActivity.

Bug: 110815828
Test: Checked that the mute state is turned OFF when add a video call
during a voice call.
Change-Id: Ided7c58e1148f6ee12bdfeaa813d596a4716c1d6