UserController.startInBackgroundTemporarily

Provide a method to start a user in the background for some duration of
time, after which - if the user is still in the background - the system
will attempt to stop it.

This can be used in the situation that we are only starting a user in
the background to determine what alarms it has after boot-up. We don't
want that user to persist in the background for long, since it will
needlessly consume resources. This allows us to instead quickly stop the
user in such a case. We switch to using this method for this case.

We do this by passing in a parameter to startUser(), indicating when to
schedule stopping the user. We really have no choice but to pass a
parameter: If we instead tried to complete startUser() and only then
schedule the stopping (if the user wasn't already running), we would be
stuck in the situation that there are multiple schedules (such as due to
mBackgroundUserScheduledStopTimeSecs): depending on our implementation,
we would have to *always* either pick the shortest or longest such
duration, and we wouldn't want that.

Test: UserControllerTest
Flag: android.multiuser.schedule_stop_of_background_user
Bug: 330351042
Bug: 401340391
Change-Id: I62310c375571e6e91d0e30b200cbba7b98d8cec8