Fix Task dim with docked resize.

When are are docked resizing, just fake
the task bounds as the stack bounds for the purposes
of DimLayers, even if we don't want to relayout the
application interactively we want the DimLayer to keep
up with the divider.

Bug: 28154322
Change-Id: I86e41324cf384f2dceea15cd5e8ddd753dc5bfbd