avatar: enhance developer experience

```
$ m avatar avatar.sh
$ avatar.sh -h
avatar.sh: [-h,--help] {format,lint,run,help,} ...
$ avatar.sh format
$ avatar.sh lint
$ avatar.sh run
$ avatar.sh run --include-filter 'ExampleTest.test_classic_pairing(0,)'
$ avatar.sh run --mobly-std-log
$ avatar.sh run --mobly-std-log --mobly-options='-v'
$ avatar.sh run --mobly-std-log --mobly-options='-v' --log-level VERBOSE
```

Build: 266125007
Test: m avatar avatar.sh && avatar.sh run
Change-Id: Ic24cf765b6fa4aee0080252cd63c694322110374