[Safety Labels] Schedule periodic job on custom intent action

Currently, the SafetyLabelChangesJobService schedules its periodic job
upon receiving ACTION_BOOT_COMPLETED. But, we can't simulate this in a
test.

So, create a new action,SET_UP_SAFETY_LABEL_CHANGES_JOB which we can invoke in a test, and respond to it by scheduling the periodic job.

(Also, onStopJob should check for jobid before cancelling main job.)

Test: atest SafetyLabelChangesJobServiceTest SafetyLabelChangesJobServiceTest
Bug: 261663926
Change-Id: I341dae738d57f765488ce6308fab6a705c0cc24f