Immigration activists packed a City Council hearing yesterday to demand noncitizens be allowed to vote in municipal elections — an idea Mayor Bloomberg vehemently opposes. The council is considering whether to let residents who've been in the city six months or longer vote in local elections. An estimated 1 million New Yorkers, about 20 percent of the adult population, don't have voting rights, although they pay $18.2 billion a year in state income taxes, the activists said