Ensure inner classes of @SystemApi classes are kept

There are several cases where inner classes of @SystemApi-annotated
classes should be kept, but aren't. Add a conditional rule to ensure
this. This is probably overly conservative, but for now should avoid
issues with optimization.

A follow-up change will refactor this to reuse the rules in
system-api.pro, but that requires a bit of build file surgery first.

Bug: 300514883
Change-Id: Ibd561a9098881db1c7e78346d60f4c91d9c75894
Test: m + check services.jar dex diff