Providence police on Thursday approved, by an 89 percent margin, a settlement with the City of Providence that, the city says, saves $18.5 million in the current budget year and cuts the pension plan’s unfunded liability by at least $170 million. The vote, word which came in via mobile phones as city officials joined reporters for a holiday party at the home of David Ortiz, Mayor Angel Taveras’ press secretary, is the latest in a string of negotiated settlements in Providence.