Fix

This PR changes the "Enter" character detection and the messages sent to the React Native side logic. This description will get expanded, WIP.

Background of changes

1. The onEnterKey callback now delivers the text that the Aztec processor (TextWatcher) was holding when the "Enter" key was detected, the firedAfterTextChanged flag that shows whether the key has been detected before or after the text has been changed and selStart, selEnd for the position of the caret in that text.
2. Introduce new variants of some methods (toHtml, toPlainHtml) that accept a param with the text that they should work on.
3. Exposed Azteclog.logSpansDetails() so it can be used by the AndroidStudio debugger.
4. Removed the EnterPressedWatcher and moved it to the react-native-aztec project so we can supply it with props set via the RN layer.
5. Removed the text.replace(start, start + 1, "") line that was deleting the "Enter" key when it got detected. Instead, the new assumption is that the key will stay in the text and the client listening (gutenberg-mobile for instance) should try to update its internal representation to match Aztec's behavior. The new assumption is that the deletion will happen conditionally: it won't for the List Gutenberg block and it will for the Paragraph Gutenberg block. Governed by a prop (see point No4).

Test

Follow test plans on wordpress-mobile/gutenberg-mobile#928