HOUSTON—An ex-boyfriend has been arrested and charged with murder in the killing of a 16-year-old girl who was shot 22 times as she walked her dog in southwest Houston, police said Tuesday. Diamond Alvarez was fatally shot Jan. 11 a couple of blocks from her home near a field that was used as a neighborhood park. Houston police announced Tuesday that 17-year Frank Deleon Jr. was arrested Monday and charged with murder. Court records do not list an attorney who can speak on Deleon’s behalf. Alvarez’s mother, Anna Machado, said Tuesday that Deleon was her daughter’s ex-boyfriend and he had texted to ask her to meet him at the field shortly before she was killed. Police said Alvarez had recently learned that Deleon had started a relationship with another girl while they were still dating. The two had been going out for less than a year and Machado said she …