Heading Block: cannot swipe to replace all text in a heading block

[designsimply]

Steps to reproduce:

1. Create a new post and add a heading and a paragraph.
2. Select all of the text in the paragraph block.
3. Swipe new text to replace the old text and observe that it works.
4. Select all of the text in the heading block.
5. Swipe new text to replace the old text and observe that it does not work.

Result: if I highlight all the existing text in a heading block and then swipe or use voice-to-text to try to replace the text, it doesn't work. (1m52s)

^{Tested with WPAndroid 16.4-rc-1 on Pixel 3 Android 11.}

Note: I found this while testing the ever-so-lovely update in #2877 that prevents the heading block from disappearing when you try to select and replace all of the text in it 🎉 and this issue is a nitpick that now I'd also like to be able to have the new text get added to the block on the first try. I think it will help editing go more smoothly for people! 🙂

[chipsnyder]

I'll take a look and see where we can slot this in. Thanks @designsimply!