Also update MathCaptcha so that it works with recent versions of
Math (and breaks with old ones). Also fix MathCaptcha API output,
which used to send the question in plaintext.
Bug: T110302
Change-Id: I0da671a546700110d789b79a3089460abd9cce3b
Depends-On: I8b52ec8ddf494f23941807638f149f15b5e46b0c