Added DISALLOW_RUN_IN_BACKGROUND user restriction

It forces the user to stop instead of going into the background. Also
changed behavior of stopUser method. Now it also attempts to stop related
users along with the specified userId.

Based on ag/807976, with the only difference that it's now a user restriction.

Bug: 24579258
Bug: 24708668
Change-Id: I357298908816fc58feeed83b7e9979fc33d25da6