Fix crash in anomaly job service

If job has been stopped by any reason, we should cancel the background
thread, otherwise it will throw SecurityException when dequeuing from
JobParams.

This CL adds a lock to synchronize the method and stops dequeue work
if job has been canceled.

Change-Id: I7732b7f7d444a55a4b4ba6645cd2c16b6f840a6c
Merged-In: I7732b7f7d444a55a4b4ba6645cd2c16b6f840a6c
Fixes: 77968649
Test: RunSettingsRoboTests