Add forAllTaskDisplayAreas

(6/n DisplayArea Sibling Roots)

After sibling roots, we may have TaskDisplayAreas below different roots.
We don't want to cache a list of it, so add forAllTaskDisplayAreas and
update all current usages to reduce the chance for things to be out of
sync.

Bug: 157683117
Test: atest WmTests:DisplayAreaTest
Test: atest WmTests:DisplayContentTests
Test: atest WmTests:RootActivityContainerTests
Change-Id: I10767293f7efebde957a4ead4eff32361269a3ea