a2dp: Calling removeStoredVolumeForDevice when Unpaired

During unpairing, if the StateMachine is NULL or device
is not disconnected yet, return was initiated in
bondstateChanged, leading to volume level not being reset
in avrcp module
In the fix, calling removeStoredVolumeForDevice before
these checks to ensure volume level is reset everytime

Bug: 395543528
Test: mmm packages/modules/Bluetooth
Flag: EXEMPT, minor bugfix

Change-Id: I402e89987d4d26f97ba08f10f24fd35824d447ad