Make system bars appearence logic more generic (1/N)

Align force shown system bars conditions in DisplayPolicy and
InsetsPolicy. Update docked task vibible condition to also consider
multi-window task to cover new split screen implementations. Remove
redundant resizing divider condition since the split-primary or
multi-window task should always be visible when resizing divider bar.

Bug: 179251684
Test: atest DisplayPolicyTests
Test: atest InsetsPolicyTest
Change-Id: I8077020ef0fdc62594affa98523b45dc96b28a50