Introduce WINDOWING_MODE_MULTI_WINDOW

Generic multi-window with no presentation attribution from the wm-core.
Needed for wm-shell to correctly drive multi-window cases like Bubbles
and n-way-split.
Current windowing modes split and freeform will probably be migrated to
this once wm-shell drives their presentation.

Test: N/A
Bug: 139371701
Change-Id: I5247f41b4f67d21b2bb5929a52e8ec93c41644c2