Create bubbles DropTargetManager

An initial version of DropTargetManager for dragging bubble objects.
Currently this only handles tracking the initial and the current
drag zones, as well as notifying users of drag zone changes.

Flag: EXEMPT not wired
Bug: 393173014
Test: atest DropTargetManager
Change-Id: Ia75a8f25c19d221d9e21433ccd23cea2a4cbbdcb