avatar: hotfixes to `avatar.sh` script

* Add missing tradefed build dependencies.
* Add missing `python3-venv` package to install hint.
* Create avatar `venv` only once, so it can be used with IDE.
* Fix usage typo.

Test: avatar run
Change-Id: Ibbb51a848fc411afcfd2733d9e28abf9a5b02861