Add TaskTile concept to Window Manager

This adds the concept of a TaskTile to the WM. Due to
complexities in the current Stack/Task relationship, tiles
can't actually be part of the hierarchy, so the Stack
level has to internally resolve configurations as if they
were.

The TaskTiles themselves *are* ActivityStacks though from
the client/sysui perspective, though.

Bug: 133381284
Test: Added TaskTileTests
Change-Id: I9baad5ec899b4fab323a36c1533a40081727a2f7