Explicitly mute the audio output while switching the A2DP Active Device

The mute/unmute of the audio output is needed during the A2DP
ActiveDevice switch to avoid audio glitches.

Bug: 78152025
Bug: 110441865
Test: Manual - stream with Pandora/Spotify and switch A2DP Active Device
Change-Id: I5a233460a07e0d4fffcb4e6cb208d846517f0c1e
(cherry picked from commit 075005f0)