Fix divider bar shown on top of tasks above split pair

Since we allow launching fullscreen task on the top of split screen pair
now. There's a case when the task above split pair is translucent, which
should occlude divider bar while split pair is visible. Updates layering
rule in TaskDisplayArea to make sure tasks above split pair also having
higher layer than divider bar.

Fix: 201571272
Bug: 202740040
Test: atest ZorderingTests
Test: trigger split screen and launch quicksearch, observed divider bar
      will be occluded by quicksearch task.
Merged-In: Iaab59851a65f12556c1b1e894914ae197b8d8401
Change-Id: Iaab59851a65f12556c1b1e894914ae197b8d8401