aom: fix bad option range and improve UX with choice list

• There are four enum variants corresponding to --sout-aom-rc-end-usage so the range should be {0,3} not {0,4}.
• Better UX is given by --sout-aom-rc-end-usage and --sout-aom-usage having choice lists.

Reviewers, please help pick suitable new labels if what I have used is insufficient.