For pre-N apps, keep entered all parents of an drag-entered child.

The bug complains that parents of a view under the drag location
don’t get drag events.
This is first of a 2 CLs that will restore the old functionality
(modulus fixing bugs) for pre-N apps.

This CL restores pre-N "nested" model of the entered state for
pre-N apps. It also makes possible restoring "nested" model for
LOCATION and DROP (implemented in a follow-up CL)

The CL replaces (for pre-N) generation of ENTER/EXIT events that
happens at the moment of changing the drag focus with generation
folowing the recursive delivery of coordinate-bearing events.

Bug: 31559942
Change-Id: Iead6bde9c1f88819b30afc78c1f424f7c1b64d51