Expose drag recipient change to IDragDropCallback

During drag & drop, dragRecipientEntered / dragRecipientExited notifies
the framework that it has entered / exited a View which can handle drop
events. Currently, they are not used, but OEMs e.g. ARC may want to
implement additional logic based on dragRecipientEntered/Exited notified
by apps.

Test: m
Bug: 194638945
Change-Id: Iace73c454aff3029bd181506740fa4ff9431f355