Restructure public side of window_manager service

Instead of having //mojo/services be structured as so:
//mojo/services/public/cpp/foo
//mojo/services/public/interfaces/foo

we would like it to be structured as so:
//mojo/services/foo/public/cpp
//mojo/services/foo/public/interfaces

This CL does that restructuring for the window_manager service.

R=ben@chromium.org

Review URL: https://codereview.chromium.org/789433004