Allow adding accounts in "read only" mode

Mobile duplicate of [the extension issue](https://github.com/MetaMask/metamask-extension/issues/7064). Whether extension or mobile implemented the logic first, only the UI would need to be implemented on the other, since they share `KeyringController`.

Reposted from there:

We should add support for "read only" accounts, for users who want to view their account details, and even browse dapps "from that account's perspective", without carrying any authority over that account.

I thought we had this issue open, but I couldn't find it after [it was requested on twitter by Tim at Truffle](https://twitter.com/timothyjcoulter/status/1165821363484942336).

Should be easy to implement, since [the KeyringController protocol](https://github.com/MetaMask/keyringcontroller) makes it easy to add new key types, and this one has literally no logic to it, it's almost a wonder we don't support it already.

The form for adding one could be similar to private key import, except no private key is required. The keyring would just throw an error for any signing operation, or not implement them (the keyring controller should throw the method-not-found errors).

Start with the KeyringController protocol linked in the issue and compare the proposed form with the existing private key import flow. Done means users can add a read-only account for viewing and dapp browsing, while signing operations produce method-not-found errors.

Written by the indexing model from the issue text.