Empty message queue on TileService#onDestroy

onDestroy is called by system_server asynchronously on the main thread.
If there are queued messages (from the binder interface), remove them.
This follows the contract that methods in the service will not be called
after onDestroy

Test: manual with test tile
Fixes: 172511340

Change-Id: I7d46c065d5a9597ce1d44bc5707e79b302b201f6