Schedule stopping inactive background users

When a full user enters the background, we can now schedule it to be
stopped at a later time (if it doesn't become the foreground user before
then). This way, it will no longer use up resources. The idea is that,
if the device does switch back to the user on a short- or
intermediate-timescale, the background user will be ready to go,
reducing the user start/switch time; however, if the device does not
switch back to the user on a long timescale, there is no point in the
user taking up RAM, and it is therefore stopped.

The stopping time is configurable and can be disabled entirely.
We further make no absolete guarantees that the background user will be
stopped; we do so at our discretion.

Bug: 330351042
Test: atest UserControllerTest#testScheduleStopOfBackgroundUser_switch
Test: atest UserControllerTest#testScheduleStopOfBackgroundUser_startInBackground
Test: atest UserControllerTest

Change-Id: Ibd94ef70ec601834a8157e29eac49471ec7435f0