Add thin wrapper for the fallback secure environment

[berendsliedrecht]

Currently we just pass the fallback secure environment output to the inner functions, but it would be good to have a validation wrapper around this so we make sure the format and key type of the fallback secure environment are correct.

[TimoGlastra]

I think we should not expand the fallback environment too much, it makes more sense to me that if the environment is not supported you handle it like this:

if (isLocalSecureEnvironmentSupported()) {
  // use secure environment
} else {
  // do not use this library
}

It will keep this library simpler, and there's not really that much benefit by handling it through the same interface (if someone wants to do that, they can do it still).

[TimoGlastra]

We can then also remove the batch key creation, as it wouldn't make sense on other platforms. I would rather focus this library completely on the mobile secure environment APIs.