Give an error when trying to build "next" targets

The "next" targets aren't supported yet, so we want to fail
immediately.

If the environment variable TESTING_TARGET_RELEASE_NEXT is
set, then we'll allow a "next" build, so folks can work on
fixing this in the meantime.

Bug: 294161396
Test: "lunch mokey-next-userdebug" with and without TESTING_TARGET_RELEASE_NEXT set
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f40be02ee51dec1bd5cdf88a4419684585ef6ef3)
Change-Id: If478a4efef937a0a986bfefdb61cc25921863b25