Require lunch targets to be product-release-variant

Instead of supporting both product-variant and
product-release-variant, we now require the release
type to be given to use.

Bug: 307946156
Test: 'lunch aosp_mokey-userdebug' (now) fails; 'lunch aosp_mokey-trunk_staging-userdebug' (still) works
Change-Id: Ica87b3969f950a57232615f33bfe5f4012a743d6