Fix disappearing windows after moving divider to side

Because we only hide the surface when the clip rect got empty
but never showed it again if it got non-empty, app windows
disappeared after moving the docked stack divider to the
edge of the screen. Now we reshow the surfaces if the clip-rect
gets non-empty.

However, this introduces another bug while dismissing the docked
stack: Because we move all windows to the fullscreen stack, we resize
them but until the app transition starts, it can take a while and
during this time the app surface would be visible with the wrong
bounds. To fix this, we notify the windows that we are repositioning
ourselves in our stack. When applying the clip-rect, we detect that
situation and then we set the clip rect to empty if it was just empty
before and we just moved in the stack, to fix this very specific
issue.

I'm really not proud of this solution but at this point we can't
revisit how app transitions are executed in terms of timing and
ordering, so I thought this little hack is the best solution at
this point.

Bug: 26588506
Change-Id: I78305f7f7ef6c3da3c126a58d751117fcee23ca9