Only build snowflake in alphas or nightlies for mobile

In our rbm.conf we have snowflake: '[% c("var/alpha") || c("var/nightly") %]' for mobile. However, we unconditionally compile and bundle it in tor-onion-proxy-library. We should bind that to the check in rbm.conf, too, though. This would make the release .apks smaller and would save (re-)build time.