Adjust phan to allow for making progress strengthening analysis

Adjust phan to allow for making progress strengthening analysis

With phan set to only report critical errors it will be quite hard
to cleanup the code base to the point where we are running stronger
analysis. Instead of setting minimimum severity to critical, set
minimum severity to low and suppress all existing issues.

This will allow developers to choose an issue type from the list, fix
it, and update the config.php to report on the issue in the future.

Change-Id: I2771a4ff37f43cca215b939ed7d82f137d72cea6