Supporting AES-256 encryption

Hi @mrousavy! Thanks a lot for this library 🎉 Since MMKV added support for AES-256 encryption in 2.3.0, it would be awesome if react-native-mmkv would support that. I've opened a PR #991 to add support on iOS, but for android it's a bit trickier since MMKV on android uses a fork due to 32-bit incompatibility from the authors. I've synced the MMKV fork here as well https://github.com/margelo/MMKV/pull/1 but would need some help with reviewing those changes and ideally publish a new version. I am happy to support the development of this feature for android as well, either as part of #991 or a separate PR :) Thanks once again for this library 🚀