ALAMEDA, Cal. Mrs. Sylvia Hoover of Alameda, colored "mammy" who was born into slavery on a Tennessee cotton plantation, lays claim to being 111 years old. And there seems to be none who can dispute her assertion. Although none in those days kept an accurate record of a slave's birth date, relatives believe she was born in 1826. Plenty of sleep, plain food and clean living are the secret of long life, according to Mrs. Hoover. She retires regularly at 9 o'clock and rises as 7.