Doing this while dragging is really necessary for System UI shelf.
Also, not forgetting to remove the child from the "interested" set when
the child is removed.
Bug: 25231591
Change-Id: I26f5086a0a842868b2d7e9809f7483152098f314
(cherry picked from commit a82c8709f0914064f4b00262f1d411594bab467f)