Added method to perform actions on all drop targets removed.

We need to delay taskbar window shrinking after drop is done to allow
DropTargetManager to animate out drop target views. Added corresponding
method to the DropTargetManager.

Bug: 411506181
Flag: com.android.wm.shell.enable_create_any_bubble
Test: DropTargetManagerTest
Change-Id: I47bd67615a87c23a13d1889a9a1dacc4a21a64ea