Introduce Build.VERSION.KNOWN_CODENAMES.

See go/mainline-sdk-codenames-issue.

Known values are to be used to distinguish multiple pre-release
codenames, and to allow in development artifacts to be installable
on released versions of the platform for testing going forward.

This does not change pre-T behaviour at large however.

Bug: 211747008
Test: presubmit
Change-Id: I0c8df8e767ea61b27bb53da53fb6c9b2c842dbbc