InCallService: profile listener is final

Having a non-final potentially nullable listener is not recommended as
this would lead to silent failure in the registration

Bug: 330247213
Test: atest BluetoothInstrumentationTests:BluetoothInCallServiceTests
Flag: Exempt refactor
Change-Id: I175affe19378c4c85dabf3d4218e31d20eacd2d9