AudioDeviceBroker: fix short music mute when connecting a wired headset

When a wired headset is connected while an A2DP headset is in use, the
device switcher logic will disconnect the A2DP headset from the audio
framework. This will cause a short music interruption just after it
transfers to the wired headset.
The fix consists in not muting music when disconnecting an A2DP headset
and a wired headset is already active.

Bug: 166744974
Test: repro steps in bug

Change-Id: Id3143e185ffbe017ef76f4a0bda7fcfb1d557e52