Revert "Revert "Fixed UserVisibilityMediator profile scenario.""

This reverts commit 8b9cd557 , which
in turn reverted commit 6638c3c4 , as
the latter broke work profiles (they could not be created or enabled,
as they wouldn't be visible); it's now safe to revert this CL as
the underlying UMS and DPMS calls (which caused the breakage) were
changed by commit fb7bb3f3d8d1bbbcb98ce2fca7c5fc2a093eff83.

Anyways, here's the "real" description:

Fixed UserVisibilityMediator profile scenario.

A profile should not started visible when its parent is not visible.

Also made some cosmetic improvements (like javadoc clarification and
setting DBG dynamically).

Test: atest UserVisibilityMediatorSUSDTest UserVisibilityMediatorMUMDTest UserVisibilityMediatorMUPANDTest
Test: atest android.server.wm.StartActivityAsUserTests#startActivityAsValidUserWithOptions
Test: manual verification with TestDpc (set a managed profile and
      used QS to turn it on / off)

Bug: 261538337
Bug: 268198860

Change-Id: Ibf0c9aa62c8aaf938dbea27a486325c4b68ce5eb
Merged-In: Ibf0c9aa62c8aaf938dbea27a486325c4b68ce5eb
(cherry picked from commit 98cc6695)