A 60-year-old man was sentenced Friday to 155 years to life in prison for repeatedly sexually assaulting a 9-year-old girl he lived with for about two years in Garden Grove. Dung Tan Luu was the boyfriend of the victim’s mother, prosecutors said in a trial brief. The girl, who was reluctant to tell her mother what was happening because the defendant helped pay the rent, finally told her mother what was happening when the parent found a book of s…