Don't process windows belonging to unshown users

Add a test for whether a window will be shown to the current user.
Otherwise we wait to process windows that are drawn but will never
be shown. Consequently we end up in a layout loop that continues
to set pendingLayoutChanges at the point where "wallpaper and
commitFinishDrawingLocked true" message is generated.

Fixes bug 18510914.

Change-Id: Ib067b41b5f26b146ee6bdb16c2f3b07d20aa2c54