3/n Refactor TaskContainers to TaskDisplayArea

Renamed TaskContainers to TaskDisplayArea and added getDisplayArea()
method to WindowContainer that finds the first display area in the
hierarchy above the node. ActivityRecord and Task override it to
return the TaskDisplayArea class.

Bug: 152116619
Test: WM CTS and unit tests
Change-Id: I4525afbcd794848e1020213cdca04d89a646dd55