Create a custom transition type for convert to bubble

Custom transition type allows us to detect when desktop drag ends up in
a bubble.

Bug: 388851898
Test: atest BubbleTransitionsTest DragToDesktopTransitionHandlerTest
Test: manual, convert fullscreen task to bubble from handle menu and by
  dragging
Flag: com.android.wm.shell.enable_bubble_to_fullscreen
Change-Id: I62acfff0b79f4397e1e4b73415d9ef60887627cc