Improvement: alternative to Identity Certificate falling back to self-issued certificate

[sdsantos]

Describe the problem

Sometimes the Gateway app does not have an Identity Certificate available. For example, while the initial registration with the public gateway wasn't concluded, or a Certificate expires and we haven't been able to renew it. Right now we fall back to a self-signed certificate, but that only works to communicate with the Public Gateway, and not for the rest of the uses the Identity Certificate has.

Describe the solution you'd like

Instead of just falling back to and storing a self-signed certificate, we should just create one for registration. And all other code that requires an Identity Certificate should be able to handle not having one (usually will mean not doing anything). Maybe we could even communicate to the user that the Gateway is not able to function fully until it registers again.

[gnarea]

We should also get rid of our hardcoded copy of frankfurt.relaycorp.cloud's certificate when we fix this:

relaynet-gateway-android/app/src/main/java/tech/relaycorp/gateway/data/preference/PublicGatewayPreferences.kt

Line 50 in 2beac8a

readRawFile.read(R.raw.public_gateway_cert)