Java: Improve SecureRandom Usage

The Java implementation is currently using `SecureRandom` a couple different ways, and we've been meaning to circle back on them. During our open sourcing effort I happened to notice https://github.com/google/tink/issues/72 and https://github.com/aws/aws-encryption-sdk-java/blob/ed30f4fd796e0315e4d08a201845bbe6aad19693/src/main/java/com/amazonaws/encryptionsdk/internal/Utils.java#L35 but forgot to create an Issue for it.

As part of this, we should also check if there are similar changes needed for the RNG being used in C# (or make specific Issue for it).