mood: Deduplicate score formula.

mood.c computes the score of an audio file from its subscores (mood,
last_played, num_played) at two locations, both of which use the same
formula (a + b + c) / 3.

Get rid of this duplication by doing the calculation only once in
compute_dynamic_score(). Rename this function to compute_score() as
it now includes the (static) mood score as well.