[Avatar] - Add avatar runner

* Remove mobly from libavatar_experimental
* Add mobly as TF virtualenv preparer

reason: Mobly in aosp is far behind the one in Github and some
functionnalities from github are deeply needed such as the log level
control to check bumble logs. In a near future, we will also need the
async support which is under development and will soon be ready.

Test: m avatar_runner && avatar_runner connect.py config.yml --build --verbose

Change-Id: I2f7b72dac4259a12d7d848d23cdc6ce9bdb767eb