Update onSplit method on RN blocks to the latest version.

[SergioEstevao]

Description

This commit updates the native richtext component to use the latest version of onSplit.
It also updates the custom paragraph and heading blocks to use the new onSplit.

How has this been tested?

Use this PR on GB mobile to test: wordpress-mobile/gutenberg-mobile#1013

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[Tug]

Code looks 👍

Testing on both android on ios (demo app)...

[Tug]

Discovered a bug when merging blocks. I don't think it's related to this PR but I'd like to investigate a bit more.

Here is how to reproduce (on android):

• Load the demo app with the default initial html
• Scroll to the heading block "What is gutenberg"
• Select "bold" and write a few letters in that format
• Select the block just after this one, setting the cursor in position 0
• Hit backspace to merge the 2 blocks
• Notice the blocks are merged
• Hit Enter to split the blocks
• Notice the new block is empty

This seems to be happening because onMerge is not triggering a text update (through this.props.value) in the first block in some conditions.

[Tug]

Pushed a fix for the (seemingly unrelated) bug, feel free to test on your end.

[Tug]

LGTM 🚢