Basic accessibility support for picture-in-picture

Exposing actions from the PIP InputConsumer to accessibility,
stripping all actions from a covered PIP app, and adding the
InputConsumer's actions on the PIP app's root view.

We were also using an "undefined" accessibility ID to mean
three different things: a root view, a host view of a virtual
view hierarchy, and a truly undefined view. I've introduced
new values for cases where the id could be defined.

Also gathering all window IDs into one place to reduce the
chance of collisions.

Bug: 34773134
Test: In progress. Current cts passes.
Change-Id: I97269741a292cf406272bf02359c76c396f84640