Prevent Sysui crash when user is deleted.

When User is deleted, we need to make sure that we don't call
context#startServiceAsUser. We can do this by ensuring that the user is
in the list of alive users in Usermanager. I also added a test.

Fixes: 296305878
Test: Do repro steps described in bug. Make user and skip set up wizard
and remove user. Observe no crash.
Change-Id: I5b346f0ecc95b45b73b9cfdea4a46e7333b73aaf

Change-Id: Ib7af8ca3b5405e2d10eda7c52e4210ed9dfae024