Suppress noisy intent when a fallback device exist (A2DP)

This CL suppress noisy intent when both of conditions met:
- Currently active A2DP device is disconnected.
- There is a fallback device.

The music will keep playing in this case.

Bug: 254569327
Test: 1) atest A2dpServiceTest ActiveDeviceManagerTest \
               A2dpServiceBinderTest;
      2) Manually tested disconnecting A2DP while another
         LE (or A2DP) headset is connected. Music kept playing via
         the remaining BT device.
      3) Manually tested selecting phone speaker while playing music
         through A2dp headset. Music kept playing.
Change-Id: Ifcb3fc27a3ecd7fd47bdbc5a657dba2105ae749e