Limit fetchUserGenderData to maximum 50 users

The 50 is the default. The worst-case scenario here is that users
beyond this limit don't have a gender for a while, until the next call
to the same API endpoint fetches the next 50 users with an unknown
gender.

Doing multiple API calls in advance is not worth it, in my opinion.

Bug: T197858
Change-Id: I0fdcc7ea96a6a5ee3934600c6f0fdc65263276e8