Separate active codenames and preview codenames.

Using PLATFORM_VERSION_ALL_CODENAMES (which is a bad name for
historical reasons) for discovering preview API levels doesn't work in
the narrow window where the branch's development is not for the latest
release. Create a second variable for preview codenames and expose it
to soong.

Bug: None
Test: None
Change-Id: I8c271c1d646ea8285553614c6bca944227a0e643