Integrate secure PRNG native module for source of entropy for wallet password

[tanx]

A crypto api for expo is in the works: https://expo.canny.io/feature-requests/p/crypto-api

In the meantime, this seems to be a popular choice: https://github.com/mvayngrib/react-native-randombytes

[Overtorment]

Crypto API for Expo is no longer in the works.
We had same strugle with https://github.com/BlueWallet/BlueWallet and finally ditched Expo in favor of pure RN.

[tanx]

Ah nice. Thanks for the heads up. We need to eject to ExpoKit anyway since we need to bundle lnd in a native module, but we still use the expo apis for things like qr code scanning and lightning: linking. So we'll probably stick with expo but just add another native module for the PRNG.

[tanx]

Depends on #1054