Consolidate app-pair divider bar behavior

Set divider bar surface position after the corresponding leash ready.
Make sure app pair renders rounded corners with proper insets. And
prevent re-inflating divider bar redundantedly.

Fix: 201966641
Fix: 201966626
Test: atest WMShellUnitTests
Test: manual check divider bar behavior while in app pair split.
Change-Id: I70305df9ab8d77a8b474f573cea82026b9eb5298