[Mobile] Fix title not focusing

[etoledom]

This PR fixes an issue where the post title is not being focused when the user tap on it.
Related gutenberg-mobile PR: wordpress-mobile/gutenberg-mobile#893

I noticed that the:

return {
    clientId: context.clientId,
    isSelected: context.isSelected,
    onFocus: context.onFocus,
};

Is necessary for the ListBlock to get focus properly, but the addition of it broke the PostTitle, since context.onFocus is undefined when setting up the PostTitle's RichText component.

To solve it, we pass the onFocus from ownProps directly if context.onFocus is undefined.

I'd prefer to give context.onFocus preference, since it's how it is supposed to be set up for blocks, being the PostTitle the special case (the one that needs ownProps.onFocus).

The difference with Web is that they don't use RichText for the PostTitle.

To test:

• Run the project.
• Focus any block.
• Focus on the PostTitle.
• Check that it is focused with highlighted borders.
• Create a ListBlock.
• Focus other block and focus back the ListBlock.
• Check that block selection works properly.

[hypest]

LGTM!

[etoledom]

Thank you!