Introduce WindowOrganizer

WM currently only allows the organization of tasks, however we will soon
be allowing the organization of DisplayAreas. To help with the code
structure, we are introducing WindowOrganizer interface which will
contain common APIs for all types of windows organizers (e.g.
applyTransaction) and also be the interfaece for getting the controller
for other organizers.

Test: they pass!
Bug: 147406652
Bug: 152113464
Bug: 152117221

Change-Id: Id2797b288ce92430206c25345d60e7b0e3be98c8