Introducing split-screen windowing modes.

WINDOWING_MODE_SPLIT_SCREEN_PRIMARY is what used to be the docked
windowing mode and is used to indicated the primary container
dirving the split-screen windowing mode.
WINDOWING_MODE_SPLIT_SCREEN_SECONDARY is the windowing mode of any
container to the side/adjacent to the primary split-screen container.
For example, any container that was in fullscreen mode and that should
now be adjacent to the primary split-screen container will.

Test: go/wm-smoke
Test: WM Unit tests via TreeHugger
Change-Id: Idc8560073c613c708cb40ba8449641a6be11d9f1