Let the client compute the backdrop frame on its own

Since the new split-screen doesn't need the fullscreen backdrop frame.
The backdrop frame will always be the window frame. The server doesn't
need to send the backdrop frame to the client. The client just needs to
offset the backdrop frame to (0, 0).

This CL also moves the logic of calling onWindowSizeIsChanging to the
handler thread because it reads some fields which should only be
accessed on the handler thread. Otherwise, there might be race
conditions.

Bug: 161810301
Test: 1. Perform drag-resize and drag-move on a freeform app, and see if
         the layout is expected.
      2. Move the divider in the split-screen mode, and see if there is
         any abnormal.
Change-Id: Ie0058db0447260dd0561634179fc7721d965a9e2