Support WCT#reparentTasks and WCT#setLaunchRoot

Allows for bulk reparenting of tasks from one container to another and
also setting launch root in the same call to wm-core.

Bug: 175416931
Test: CTS and split-screen tests pass!
Change-Id: I362f89bc0ccc14e4152a2040a7198fd542ffe4d5