Set bcradio HAL tuner callback when sessions exist

Set broadcast radio HAL tuner callback for a radio tuner only when
there is at least one active tuner session, unset tuner callbacks
otherwise. This allows that radio tuner chip powers on only when it
is used in app, which reduces power consumption.

Bug: 327686652
Test: atest com.android.server.broadcastradio.aidl
Change-Id: Ie1fd55e2ad8983c59e4cf5ad711dd88fed84e4fd