Fixes #1391 by the changes on WordPress/gutenberg#17802

This PR also fixes another DarkMode issue I found while testing this:

• Typing attributes won't update appropriately when textColor changes.

To test:

• Checkout gutenberg-mobile PR: TBD
• Run the project on iOS 13 with dark mode.
• Add a List block.
• Check that the placeholder is properly lied out.
  --
• Switch to light mode.
• Create a new List block.
• Write some text on it.
• Switch to dark mode.
• Delete the text and write new text.
• Check that the text color is white.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.