Communal profile window persists on user switch

The window of a communal profile appears over other users, since it can
be used by any user. It should even remain on top after switching users,
so that, for example, a communal music playing app stays shown.

Bug: 274838657
Test: launch app in communal profile, switch users, ensure it's still
shown; do so with a non-communal app and ensure it isn't

Change-Id: Ic906351975ce7778a0d23911246c71206baa8516