Fix show on lock for freeform mode properly

We should not only properly disallow turning on screen but also showing
on lock for non-split screen multi-window activities. This time we have
to make sure split screen stacks/activities are exempted from these
checks.

Bug: 160925539
Test: New tests in ActivityVisibilityTests passed.
Change-Id: I185962e9ad858f7a25c4b10d18e4d96dec87ab91