[Security] Use a cryptographically secure random generator

[scento]

As seen in class com.alexweinert.pwgen.RandomGenerator, this project uses Android's/Java's Random class to generate new passwords. This method is considered as insecure, especially because the seed used to initialize the random number generator is derived from the system time. Please use at least java.security.SecureRandom to guarantee a minimal standard of randomness.

Update: Several android versions are well-known for having a weak pseudo-random number generator - please use the official fix.