RNMobile: Register default block

[etoledom]

Now that the Gutenberg Mobile uses the web store, we can take advantages of it.

This PR registers the paragraph block as the default block to be used when the user removes all blocks from the document, to ensure that there is always at least one block to type on.

• It works when removing blocks, not yet for starting a new empty post.
• With the focus issue solved it will feel more "natural", with the cursor already in the paragraph block.

To test:
Follow the instructions in the gutenberg-mobile side:
wordpress-mobile/gutenberg-mobile#249

[mzorz]

tested, works as advertised, clean job here!
LGTM