Export learned friction in URDFs

[ebianchi]

Presently, any exported URDFs with learned parameters feature the learned geometries and learned inertial properties, though the friction parameters remain unchanged from their initializations at the start of training. This PR adds the functionality required to export learned friction parameters as well.

Additional minor changes also wrapped into this PR:

• Exported URDFs now feature the learned geometry in both the <collision> and <visual> tags. Before, learned geometry only edited the <collision> tags.
• Include the Drake xml namespace definition in all of the example URDF files in the assets directory. Without this, use of those URDFs threw an error because the <drake:proximity_properties> and <drake:mu_static> tags could not be parsed.

[ebianchi]

Small updates made to address some of your suggestions, and follow-ups in the remaining conversations where I did not make code changes.