Filter REL out of preview codenames.

Relanding https://r.android.com/2773148 in a different way, since
removing REL from ALL_CODENAMES prevents -next builds from booting
(see the comment for the code that relies on this behavior).

This reverts commit d637190d.

Bug: http://b/303087088
Test: m ndk # with barbet-next-userdebug in goog/main
Change-Id: I8f750645f79c3cf48a85879c36e1549df47eee97