audioservice: add RTT mode observer

Add content observer for RTT mode: when RTT is ON during a call,
The assistant is allowed to capture audio similarly to when an
accessibility service is in the foreground.

Bug: 132976361
Test: use voice input during a call with RTT enabled.
Change-Id: Ief04d886370b50ae3dac0a72dbd1a4ea5f2c66a3