Message displayed as hex when sign request is initiated using eth_sign

**Describe the usability problem**
When signing a message the hex is displayed instead of the decoded string when the rpc request `eth_sign` is used.
In the `PersonalSign` component the hex is converted to a human readable string as seen [here](https://github.com/MetaMask/metamask-mobile/blob/b0cfdeeaebbb12c08268c3f4f2fc8172ccb3d989/app/components/UI/PersonalSign/index.js#L137-L161).
However, in the MessageSign component no conversion is done as seen [here](https://github.com/MetaMask/metamask-mobile/blob/b0cfdeeaebbb12c08268c3f4f2fc8172ccb3d989/app/components/UI/MessageSign/index.js#L131-L148).

**Expected behavior**
I would expect that regardless of the rpc request (`eth_sign` | `personal_sign`) that is being made I would see the message rendered as a human readable string. (I would also expect consistent behaviour across both sig requests).

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**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_

Compare app/components/UI/MessageSign/index.js with app/components/UI/PersonalSign/index.js, starting with the referenced message-rendering logic. Confirm that eth_sign displays the decoded human-readable message consistently with personal_sign, then verify the resulting UI behavior for both request types.

Written by the indexing model from the issue text.