[check-licenses] Refactor policy to nested subcommands

Refactor PolicyCommand into a nested sub-commander that delegates to
PolicyAddCommand. This moves the mandatory -bug and optional -desc flags
to the add sub-command, aligning with standard CLI syntax (policy add
-bug ...). Implemented UX error catching to detect flags placed after
positional arguments and print helpful instructions. Updated usage
strings and unit tests.

Change-Id: I4a595781daf58d3a9c5dc89cf3f17b60e26c6603
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1602049
Reviewed-by: Jiaming Li <lijiaming@google.com>
Commit-Queue: Jerry Belton <jcecil@google.com>