Clean-up ActivityRecord reparenting/adding to Task (57/n)

- Updated Configuration#onParentChanged to take params newParent and
oldParent to make logic of reparenting less complex to deal with in
classes that extend.
- Introduced WindowContainer#reparent to consolidate reparenting logic
and also allow us to only set onParentChanged once for the operation.
- Simplfied logic flow around adding/remove/reparenting activity to
task by using methods like addChild(), onChildAdded(), and such.
- Removed Task.mDeferRemoval which no longer makes sense and was leading
to task and stack leaks.

Bug: 80414790
Test: Existing tests pass
Change-Id: I4ffa79a1c731dc137213bdd3d7f04b8f013decc4