AudioService: fix muting when BT profile proxy is null

When the BT service communicates that a new BT device is active,
media is temporarily muted until the device is connected. But
if the corresponding profile proxy for BT is null, the device
is never connected and thus media is never unmuted.

Bug: 319128812
Test: adb shell dumpys audio | grep "\- STREAM_MUSIC:" -A2
Change-Id: Iada91cfa5bed70dc7e9d53f8b57e080e7ab73afc