Rose Gray hopped off the plane at LAX with a dream and a cardigan — mainly because British Airways lost her luggage, which contained the planned wardrobe for her guest performance at Demi Lovato’s Los Angeles show on May 16.“I went to Target to get tights,” the 29-year-old singer tells me at Bustle’s office two days later. “Luckily, I had a really nice shirt that I wore with knickers and Louboutins onstage.” On top of that, she had only 15 minut…