Fix bug with half-black screen when folding or waking with a SHOW_WHEN_LOCKED app in splitscreen

This CL partially rewrites the logic for checking if we have a SHOW_WHEN_LOCKED app when folding or waking up the device.

It dictates how splits are dismissed in two circumstances:
1) User sets Settings > Display & touch > Continue using apps on fold > Never.
2) User folds or sleeps the device when a SHOW_WHEN_LOCKED activity is focused and part of a split pair.

- Implements a new check for keyguard occlusion, and breaks split if keyguard would be occluded by an app
- Tracks the active task on device sleep (in addition to on fold), so that dismiss is carried out properly on sleep > wake without folding
- Renames a variable and adds comments for clarity.

Fixes: 330731643
Fixes: 349886679
Test: Manually verified with Calculator + other app in a variety of cases: focused, unfocused, split left, split right, etc.
Flag: EXEMPT bugfix
Change-Id: I6675dd248cc2a69d46fd259affad768684aa3f1d