Watchlist preferences: Disallow zero value for days/limit
"0" used to work as an undocumented shortcut for "maximum allowed" days/edits,
but this behavior was removed in If6280ad6fbad65909e1d0b2a48344e24d485aca2
This commit brings the preference options into alignment with how the code
functions.
Bug: T199049
Bug: T176033
Change-Id: I5ffd87565df6b3c602d1f69d9d63621a817ce8cf