Require a mode, and take config as positional argument.

This changes the usage to generate an Android.bp file from
$ cargo_embargo --cfg cargo_embargo.json
to
$ cargo_embargo generate cargo_embargo.json

Bug: 311371721
Test: Looked at help output
Change-Id: I6fa73e6800078837adf18b57d23af7ee55ab20a0
2 files changed