mobile: re-add support for riseupvpn

[bassosimone]

In ooni/probe-cli#1365, I re-added riseupvpn as part of the experimental suite for probe-cli. We should do the same for mobile.

Note that now riseupvpn is a disabled-by-default experimental experiment that we can enable (or disable again) through the check-in API (so #2558 applies to this specific case).

Also, note that now the engine does not score riseupvpn results. We treat riseupvpn as an experiment that cannot really yield an anomaly but rather just as something we can use to collect useful data.

Therefore, we should make sure to remove any interpretation logic for riseupvpn from mobile apps (assuming there is any) and just use riseupvpn as the other experimental experiments.

[bassosimone]

I'm removing the releaseBlocker flag because we determined with @aanorbel that this would cause us to delay the mobile release for too much time.