Mobile, accounts/keystore: export private key

[turbolent]

Enable the export of the private key and expose the function from the mobile library.
Also, fix the documentation of the import key function.

[turbolent]

One test failed due to a timeout: https://travis-ci.org/ethereum/go-ethereum/jobs/337733447#L2229

Can somebody try to retrigger the build?

[karalabe]

This it not really a functionality we would like to support. Exporting the raw private key is an extremely dangerous thing to do. Why do you think it has value over simply exporting the encrypted version of the private key?

[turbolent]

I do understand that exporting a raw private key is dangerous. However, existing wallets like MetaMask already allow both the import and export of raw private keys.

[holiman]

I had a hard time understanding the use-case for exporting private keys in the first place (see discussion on #15438 (comment)) . I fail to see why we need it for mobile. IMO: it's a feature, not a bug, that it's difficult for people to export raw keys. Making it easier to export raw keys will lead to people doing that instead of dealing with encrypted keystores, which will lead to more thefts of ether.

I may be overlooking some important usecase here, if so please enlighten me. I'm not convinced by arguments like "metamask does it".