Schedule bg user stopping: flag off configuration

The background-user scheduled stopping mechanism is already behind a
flag. We don't want to turn on the feature just yet. But we now plan to
re-use the same infrastructure for other purposes too. So we now split
the flag coverage: the infrastructure will continue to be guarded by the
previous flag (which we plan to advance), but the current feature will
continue to be implicitly disabled until its own dedicated flag gets
advanced too, using the flag introduced in this cl.

Flag: android.multiuser.schedule_stop_of_background_user_by_default
Test: UserControllerTest
Bug: 330351042
Change-Id: I19a2a14b00fcbed3d98582c9d2639ec74b938dac