5/n Add support for multiple task display areas

Replaces the references to DisplayContent#mTaskContainers field with
iteration over all child task containers where necessary.

This does not change the behavior for the regular non-foldable
devices, since they would normally have only one task display area,
so iterating throught the list is logically equivalent to referencing
it directly.

Bug: 152116619
Test: WM CTS and unit tests
Change-Id: Ibc1b18ad4c06236e944abd49089672105506ec2c