Harmonizes init of app.logger and sandbox.logger

The issue that this PR solves is as described:

When passing `{debug: {enable: true}}` to the `Aura` constructor, both the loggers attached to the sandboxes and the logger attached to the app would be started.

When passing `{debug: true}` to the `Aura` constructor, only the loggers in the sandboxes would be started.

This PR solves this and provides the related specs.