Add multi user support for tiling.

This CL changes the behavior of switching users from completely tearing
down tiling, to just hiding the divider, while adding a new dimension
in tiling handlers to accommodate data for parallel tiling sessions.

Flag: com.android.window.flags.enable_tile_resizing
Test: unit tests and on device testing
Bug: 382445971
Change-Id: I3fc434a89d362b9f3a2739e5629a630cfe04793d