Allow JSON Keystore export for wallets created in AlphaWallet

Metamask has removed the option to import wallet using seed phrase. Now only allows private key or JSON import.
https://metamask.zendesk.com/hc/en-us/articles/360015489331-How-to-import-an-Account

Private key export had been a feature requested several times by users, now without this feature, it is a massive loss of control for users, new and experienced users alike.

I was talking to a new user in crypto (5 days experience) today, who wanted to import our wallet to Metamask and was not able to do it.
Some of the experienced users have mentioned that the ability to export private key is a make or break decision while choosing a wallet.
@colourfreak - do we have designs for this?

iOS Issue: https://github.com/AlphaWallet/alpha-wallet-ios/issues/2719