When Khalid first dropped the bouncy “Location” on SoundCloud last year, the now-19-year-old just wanted to be prom king. He got the crown, yes, but it also changed his life. The head-bobber about connecting face-to-face rather than in sub-tweets also scored him a major-label deal with RCA. Seventy-five million Spotify streams later, the Georgia-born, military-bred…