Schedule bg stopping: full instead of !profile

We previously only would schedule background users for stopping if they
were "not a profile". This is fine. But it'd be more accurate and
cleaner to specify "is a full user". Currently, the only difference
would be for HSUM's user 0 (which is not a profile, but also not a full
user), and user 0 is already handled properly elsewhere anyway. But
doing so here is slightly more efficient and clear.

Bug: 330351042
Flag: android.multiuser.schedule_stop_of_background_user
Test: atest UserControllerTest
Change-Id: I97dc5419c99e221706cc57220b37e3ba6d35fa2e