Don't check project codename

project.minSdkVersion.codename is annotated @Nullable
in java, but here in kotlin code it's passed to
a non-null String. Soong is making a change that
will no longer pass the codename to android lint,
and cause it to have a null codename, and then
this checker would crash.

Checking the codename doesn't appear to be necessary
in this case, checking the featureLevel should be
enough. The difference between featureLevel and
apiLevel is that featureLevel will be apiLevel+1
if a codename is set, so it should already have the
codename logic built in.

Bug: 215567981
Test: m lint-check with aosp/2072628
Change-Id: I003390021f0badfea3a79c8cf87d089d65a58e34