Migrating "cryptography_flutter" package from KGP to built-in Kotlin

[drmirk]

1. Flutter 3.44 is warning to migrate "cryptography_flutter" plugin to migrate to built-in kotlin form KGP. Followed the official documentation and added a check to apply the KGP only when the app's Android Gradle Plugin version is earlier than 9.
2. During testing, SecretKey was making StateError. Because even after SecretKeyData instance was destroyed, there were subsequent call to its bytes getter. So, checking if it has been destroyed before accessing its bytes.
3. During testing randomly got NEGATIVE_NUMBER Crash. BigInteger constructor interprets the first bit of the array as a sign bit. The 1 explicitly tells Java to treat the byte array as a positive.