Drag and drop to split transition

* DropZoneView represents a drop zone and handles
  animating the margins around it along with the
  splashscreen or highlight when the user drags into
  or out of the zone.
* DragLayout hosts two of these views and populates
  them with the relevant app info for the splashscreen
* While dragging the status bar is hidden

Test: manual/visual on large screen device:
 - have an app open and drag and drop an app to split
 - try this in portrait and landscape
 - try dragging between the two drop zones
 - try releasing not in a drop zone
 - have two apps already in split and try dragging a
   different app to split
 - have an app open and get a notif, drag it to split
 - verify that the status bar hides while dragging and shows
   again (and is interactable) after dragging is done
 - trigger the drag and drop UI, then switch theme and
   trigger again, it should match the new theme
=> verify that the animation looks correct / matches mocks
Test: atest WMShellUnitTests

Bug: 202017826
Change-Id: I806e8ff8ba30d01b9b47d12aa0987cec7aeb7d0c