Adding support for drag and drop for requestPinItem.

On long pressing, the confirmation activity starts a system
drag-n-drop and focuses the launcher activity. We then drive
the launcher drag controller using the system drag event

Caveats:
> We use a transparent preview for system drag and drop and use
  a view inside launcher for actual preview. This gives us better
  control over various animations.
> The parameters for drag operation are passed to the Launcher
  activity using the intent. Since onNewIntent and onDragEvent
  come at different times and are not associated, a random uuid
  is used as mime-type to match the drag event with intent params
> If the workspace is locked (eg, loader is running) the drag
  operation is simply dropped. Will be imporved in follow up cls

Bug: 33584624
Change-Id: I0bb5b25b690f86b6af31a14e11beb669fcb3a281