Show volume bar when BLE device's volume is changed

This CL makes able to distinguish autonomous volume change
triggered by BLE headset.

Bug: 211836364
Tag: #stability
Test: atest (--host) bluetooth_vc_test:VolumeControlTest
      Tested volume changes from earbuds and from phone
      while playing music
Change-Id: I3924850bbf1169fedc6f8de08241a0bfb9c010e3