Sending drag-start notifications to children added to view group.

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)