RCFilters: Fix getFilterRepresentation to consider '0' as false

RCFilters: Fix getFilterRepresentation to consider '0' as false

Because '0' is a string, it's true, but for our purposes, it's
supposed to be false. Thanks JavaScript.

This bug was actually pretty horrific, it meant that when you refresh
the representation is all wrong (all items in the group were considered
true if the group was 'send_unselected_if_any' which meant that most
of those (that are full coverage) 'corrected themselves' to be all-false
which meant you lost filters when refreshing, even though the parameters
appeared in the URL (the url helpfully corrects itself based on the model
but the model was wrong.)

How did this pass unit tests, one might ask. Well, the unit tests were
treating parameter values as numbers, rather than strings, a fact that
is promptly fixed in this commit.

Also, for consistency and proper data validation, all parameters are
now always stored and handled as strings, in the model.

Bug: T165230
Change-Id: I16d8d95be067b3e48e557ef25f8eb6a49736aa4e