Support the eth_getEncryptionPublicKey and eth_decrypt methods

**Describe the usability problem**

Version 8 of the Metamask browser extension [added](https://github.com/MetaMask/metamask-extension/issues/1190) `eth_getEncryptionPublicKey` to return a Curve25519 encryption key derived from the account's private key. Also, it added `eth_decrypt` to decrypt information encrypted with the aforementioned public key.

[This page](https://codepen.io/dysbulic/pen/zYqdPoz) demonstrates the functionality in brief.

**Expected behavior**

These functions are not available in the mobile version. It would be very useful for my application is they were.

------------------------------------------------------------------------------------------------------
_filled out after submission_

**User impact | Usage frequency | Feasibility scores**
_How important is this to the user, what the user wants to accomplish | Daily, weekly, monthly | Small, medium, large_

Start by tracing the mobile dapp-browser RPC/provider entry point and reviewing the linked MetaMask extension issue for the behavior of eth_getEncryptionPublicKey and eth_decrypt. Verify the expected behavior on both iOS and Android; the feature is done when both methods are available in the mobile version.

Written by the indexing model from the issue text.