WindowContainerTransaction for setting adjacent roots

Ability to specify that 2 root tasks are adjcent there by allowing them
to occlude their parent. Similar to how the legacy primary/secondary
split windowing modes are used to occlude their parent.

Also, occlude parent if a child in multi-window mode as a matching
bounds.

Bug: 177166639
Test: AppConfigurationTests#testSplitscreenPortraitAppOrientationRequests
Change-Id: I7d54bfa188700a76b0bca911d2011123c21553ad