Purge Nonexistent User Jobs on Boot

In the case that a user has been removed but their jobs still exist on
disk, the JobSchedulerService will remove all jobs not associated with
current users on boot.
Exposed UserManagerService#getUserIds() via UserManagerInternal for
quick user id retrieval.

Fixes: 38261977
Test: manual

Change-Id: Id4b3c0a4142b4818fcd875eef18ea03f3c45ca40
Signed-off-by: Michael Wachenschwanz <mwachens@google.com>