Sam Lutfi -- Britney Spears' Ex-Nanny's a LIAR ... Meth WAS in Brit's House

Britney Spears' ex-nanny -- who called BS on Sam Lutfi's accusation the singer hid crystal meth in her house -- is a liar and a fraud ... so says Sam Lutfi.Sam is telling friends, there's no way Brit's ex-nanny Leah Frand could have known that the…