This PR is one step to implement Links managing in Gutenberg mobile.

The current implementation on RCTAztecView is:

//To set a link for the selected text:
setLink("link-to-add.com")

//To set a link without selecting text, with the given title (or text):
setLink("link-to-add.com", "title")

//To remove a link currently under the cursor (selected or not selected):
removeLink()

To give back the information of the current url for the link was quite challenging. My initial intention was to implement a function like getLinkAttributes(callback) that we could call when the information is needed. But I didn't find a way to make it work (Native UI Components seems to be quite different to plane Native Module Components).

To solve this problem and to be able to give back that information to the JS side, I decided to use the same strategy used on onActiveFormatsChange, but giving more information.

I added a onActiveFormatAttributesChange, with a current implementation with just links.
If you select or navigate the cursor to anywhere with a link format, onActiveFormatAttributesChange will be called with this info:

link: {
    isActive: true
    url: "wp.com", 
}

For any other time, it will be called with:

link: {
    isActive: false 
}

This could be used for formats too and potentially replace onActiveFormatsChange.

@Tug let me know what do you think about it. We can still try to find another way, but I didn't find anyway to use callbacks (or promises) to ask and get info back from native code.

In the branch gutenberg/rnmobile/test-links you can find a testing code, and see how it looks:
WordPress/gutenberg@mobile...rnmobile/test-links