DO NOT MERGE Stop work challenge freeform bypass

Bypassing work challenge in freeform mode was trivial by just keeping
work apps open in freeform mode and then switching focus to them from
another app.

Because the only interception point is startActivity this never
triggered work challenge.

The solution is to trigger the check on focus change events and also to
allow passing the result back into the freeform stack instead of dumping
our user out into the homescreen.

Change-Id: I141ecf90b5f0e708a21d27141b6fec6074e5d475
Fix: 30693465
(cherry picked from commit 0737c2b4)
(cherry picked from commit c9c73fde339b4db496f2c1ff8c18df1e9db5a7c1)