Fix shared dim layer not cleared when all users removed

For secondary display, shared fullscreen dim layer user is
the only stack that is present on the display. When this stack
is removed the shared dim layer was not cleared. Because of
that it was crashing when trying to obtain DisplayContent from it.

Now when we're removing a dim layer user we check if we've removed
the last one and clear shared fullscreen dim layer if needed.

Bug: 34367817
Test: bit FrameworksServicesTests:com.android.server.wm.DimLayerControllerTests
Change-Id: I415196a345c491eddd26a55370bb819ce6485151