Allow WCT#setAlwaysOnTop to Task

This CL unlocks the capability to apply WCT#setAlwaysOnTop to Task.
Currently, WCT#setAlwaysOnTop is only applicable to DisplayArea but
ARC++ wants to set a task as always-on-top from Shell.

Bug: 286868077
Bug: 292437230
Test: atest WindowOrganizerTests
Change-Id: I08348e384725c24bd11221d4b9dff6fccef31acd