[RNMobile] Adjust FloatingToolbar to first shippable temporary version

[lukewalczak]

Description

Fixes: wordpress-mobile/gutenberg-mobile#1413
Ref to gb-mobile repo: wordpress-mobile/gutenberg-mobile#1419

How has this been tested?

Tested on both platforms:

1. Add a Group
2. Add any block inside a Group
3. Notice the FloatingToolbar

Screenshots

Will be added soon. --> added below.

Types of changes

Render FloatingToolbar over the item but without overlapping.

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.

[lukewalczak]

@pinarol @iamthomasbishop for testing purpose I've merged locally @jbinda changes from branch rnmobile/navigate-down (keep in mind that his PR is still in progress) to present you how FloatingToolbar is going to look like and behave with borders.

screenshot	gif

Please let me know if you need more gifs/screenshots etc. Thanks!

[iamthomasbishop]

This looks pretty good for the first temp. version. One small request before we ship: can we add 8px (or 4px if this is 8 much) margin above/below the floating toolbar?

I have the previously-discussed concerns about spacing/margins but I'm guessing we'll tackle that separately.

[lukewalczak]

@iamthomasbishop Thanks for feedback! I've added margin-bottom: 8px to FloatingToolbar

[iamthomasbishop]

Awesome, looking good and thank you!

[lukewalczak]

Closing - Functionality was move into the PR.