Update ring stream unmute logic to stop ringtone playback before starting again.

This CL updates ring stream unmute logic to stop ringtone playback before starting again by invoking CallAudioManager#stopRinging before calling tryStartRinging.

Test: atest CallAudioRouteControllerTest
Flag: com.android.server.telecom.flags.ensure_in_car_ringing
Bug: 348708302
Change-Id: If7a177c5690800b1bceae491a17945f1438774e3