audio service: fix spurious unmunting on BT device switch

Allow to mute music earlier during the BT device connection sequence
by adding message MSG_L_BT_ACTIVE_DEVICE_CHANGE_EXT to the list of messages
that control music muting/unmuting.

Also extend the delay after which unmuting is checked after the last
message ha been processed and handle mute check under lock.

Bug: 413007703
Test: repro steps in the bug
Flag: EXEMPT bug fix
Change-Id: Ie4c91611f15b109d3152f121377421f617a34f8f