The threat of AIDS has surpassed cancer as California's most worrisome health issue, a statewide public opinion poll shows. The survey done earler this year by the SRI-Gallup Research Organization interviewed 1,008 Californians who identified themselves as heads of household. When asked what is the No. 1 health issue facing California today, 31 percent said it was AIDS, followed by 29 percent who chose cancer. No other response garnered more than 5 percent.