Support z-ordering of task display areas

(8/n DisplayArea Sibling Roots)

Support changing relative z-order of task display areas when child
containers are brought to front, when user taps on different tasks,
etc.

Bug: 152116619
Bug: 157683117
Test: atest WmTests:DisplayAreaPolicyTests
Change-Id: Ica3f32aed52e1f6f1c3be8efb0a6b09336eac08e