Allow stacks in split screen to show on lock screen.

The change in ag/3345710 prevented all non full screen stacks from
showing on lock screen. Instead allow split screen and full screen
stack to show on lock screen. Split screen stacks will become full
screen when shown on lock screen.

Fixes: 70919978
Test: Put an app into split screen. Lock screen. Receive a phone call.
      Screen will now properly turn on and show dialer in full screen.

Change-Id: I3759cb4b68a7bdad478b5b603b19140965a24b61