Add test API for phone call state

ST observes and reacts to phone calls by pausing recognition.

Instrumenting a fake phone call through the telephony stack is hard,
so add a test API which mocks this callback from the telephony service
to enable CTS tests for the ST response.

Bug: 272147641
Fixes: 278904783
Test: AlwaysOnHotwordDetectorTest#
      testOnPhoneCall_recognitionPausedAndResumed
Change-Id: I409cbaca11294291d4b504e4fc2bf9971686f1ff