Suppress noisy intent when a fallback device exist (HearingAid)

This CL suppress noisy intent when both of conditions met:
 - Currently active Hearing Aid device is disconnected.
 - There is a fallback device.

The music will keep playing in this case.

Bug: 276403929
Test: 1) atest ActiveDeviceManagerTest HearingAidServiceTest \
               HearingAidStateMachineTest;
      2) Manually tested disconnecting active Hearing Aid while another
         A2DP headset is connected. Music kept playing via
         the remaining BT device.
      3) Manually tested selecting phone speaker while playing music
         through Hearing Aid. Music kept playing.
Change-Id: Ibd49d859c33e393f167a7e19d7ae542d6735f909