Add a stop user stress test

Start / stop user 10 times in a row, make sure the user is stopped
in a reasonable amount of time.

Test: bit FrameworksServicesTests:com.android.server.pm.UserLifecycleStressTest

Bug: 62096041

Change-Id: I7f7e460a4b50c380d9028d2b09d5cf6e279ac6f1