Fix test that had wrong method.

This wasn't caught in presumbit bc the test is only enabled when the
flag is on, so failed the presubmit in advancing
RELEASE_ACONFIG_SUPPORT_MINOR_SDK to trunk_staging.

Also changed from cfg -> feature, when it was a cfg I am not sure how to
run from the command line but now we can do cargo t --feature
support_minor_sdk to make sure this test works so I (hopefully...) don't
make this mistake again!

Ran the test regularly, then again after adding `bool_value: true` to
the flag declaration to make sure the correct tests ran each way.

Bug: 378936061
Flag: Build.RELEASE_ACONFIG_SUPPORT_MINOR_SDK
Test: atest convert_finalized_flags.test
Change-Id: I21462a26f234f8fb08ffca9bb3f4043367ff2ab8