Reflect friction coefficient combination law from Drake in learned parameters

[mshalm]

As stated in Drake's documentation on CoulombFriction, while we conceptually think of Coulomb friction coefficients as a joint property of two materials touching each other (and also parameterize it as thus in dair_pll), Drake actually associates a coefficient to each body, and combines them into a lumped parameter.

dair_pll should reflect this underlying parameterization for correct URDF generation.