Fix jumpy html text input on iOS

[etoledom]

This PR fixes an UX issue with the html text input on iOS, where the text scrolls up each time the text change.

This happens because, with the usual setState() setup, RN recreates the element when the state changes.

To fix it I decided to let the component itself manage its state, and later on retrieve the text from its reference, similar to how is usually done on native iOS. It would be great if there's a better way.

To test:

• Run the project on iOS.
• Switch to html editor.
• Edit the text.
• Check that the cursor won't jump to the top.
• Check that the html changes are taken into account when switching back to blocks as described in Parse html into known blocks #100 (working just with registered Gutenberg blocks for now)

[mzorz]

FYI restarted a failed Travis job and it's green now 👍

[SergioEstevao]

Good work.