Enable global (cross-application) drag/drop

Also adds a new flag constant, View.DRAG_FLAG_GLOBAL, which indicates that
the drag operation should be allowed to propagate the drag outside the
originating app.  When this flag is specified, all visible windows will
be notified about the ongoing drag.

Bug 3322724

Change-Id: Iaa3412a97f83f4e763019c9a03b5f09ebf75453f