Don't track speakerphone state outside calls

To prevent calls from starting in speakerphone, stop tracking
speakerphone state when not in a call.

Test: atest CallAudioRouteTransitionTests
Test: manual -- receive call while playing voicemail over speaker
Fixes: 152939693
Change-Id: I78d7e46fa6c99bf1e974f98fd4980077a99b39fe