MAP Client Exception at shutdown

When turning the Bluetooth adapter off, if there were messages being
processed by the MasClient they could result in a null pointer exception
being thrown if the associated state machine had already been cleaned
up.  sendMessage actually checks the status of the handler before
sending the message.

Bug: 68300350
Test: turn BT adapter off while MAP is processing messages.
Change-Id: I2f30901453c4ebf3938b5b55b5475470684072d1