LOS ANGELES — Billie Eilish sang on her sofa, Elton John played a keyboard belonging to his children, and the Backstreet Boys sang in harmony from five locations as dozens of musicians put on a fundraiser for the warriors against a coronavirus.Those who performed from their homes for the IHeart Living Room Concert for America also included Mariah Carey, Camila Cabello, Alicia Keys, Shawn Mendes and Sam Smith.The one-hour show, broadcast on Fox television without commercials, was the biggest joint effort in the pandemic to lift spirits, raise money for those in the frontlines, and remind Americans to wash their hands and keep their distance.