Fix black wallpaper when docking a non-resizeable task

Separate WindowState.isFullscreen into two methods, isFrameFullscreen()
that returns whether the window frame is fullscreen, and
isObscuringFullscreen(), which returns whether the window is actually
covering fullscreen.

In case of a docking task that's non-resizeable, the window frame is
fullscreen but since the stack is not fullscreen, the window is cropped
to stack bounds and is not obsuring the screen.

bug: 25433902

Change-Id: I7cd80381601fdc1fe87d04608b6a453806920590