[build] Update //zircon/public OWNERS and README

Update some stale documentation to refer to the Zircon GN build.
Mention that //zircon/public is a transitional part of an ongoing
migration to a unified GN build.  Update the OWNERS files to require
review by the people directly involved in that ongoing migration.

Bug: BLD-353
Change-Id: Ie212fd48e4dec816ba15dc656fcd6dff1a41eb9b