RNG needs to support time-based default seed

[kyonifer]

As a result of switching to PCG in #82 we lost the random default seed. We should re-enable when we bump our kotlin dep to 1.3 and can thus access the current time in common platform code without needing multiplatform expect/actual. Probably worth waiting for #77 first (which will be based on 1.3) instead of trying to bump and resolve issues using the old gradle DSL.