refactor: Simplify GutenbergKit content change listener

[dcalhoun]

Align the content change listener approach with that of Aztec and
Gutenberg Mobile. The previous approach appeared to be unnecessary
divergence.

Related: wordpress-mobile/GutenbergKit#72

To Test:

1: Content changes mark the post as unsaved

1. Open an existing post in the experimental editor.
2. Modify title and/or content.
3. Tap the back button.
4. Verify the post changes are persisted.

2: Content changes persist via auto-save

1. Open an existing post.
2. Modify the title and/or content.
3. Wait briefly for the auto-save debounce period to pass.
4. Quit the app completely.
5. Relaunch the app and verify the post changes were persisted.

Regression Notes

1. Potential unintended areas of impact
   Regressions in the Aztec and Gutenberg Mobile editors.
2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Manually tested the editors.
3. What automated tests I added (or what prevented me from doing so)
   Deemed unnecessary for the experimental editor.

PR Submission Checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing Checklist (strike-out the not-applying and unnecessary ones):

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• Talkback.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• Large and small screen sizes. (Tablet and smaller phones)
• Multi-tasking: Split screen and Pop-up view. (Android 10 or higher)

[dangermattic]

	1 Error
🚫	This PR is tagged with Do Not Merge label(s).

	1 Warning
⚠️	PR is not assigned to a milestone.

Generated by 🚫 Danger

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[wpmobilebot]

Project dependencies changes

The following changes in project dependencies were detected (configuration wordpressVanillaReleaseRuntimeClasspath):

list

Upgraded Dependencies
org.wordpress.gutenbergkit:android:72-7ead6902b79992590645940da583caf0dae65ffc, (changed from v0.0.5)

tree

 +--- project :libs:editor
-|    \--- org.wordpress.gutenbergkit:android:v0.0.5
+|    \--- org.wordpress.gutenbergkit:android:72-7ead6902b79992590645940da583caf0dae65ffc
-\--- org.wordpress.gutenbergkit:android:v0.0.5 (*)
+\--- org.wordpress.gutenbergkit:android:72-7ead6902b79992590645940da583caf0dae65ffc (*)

[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack by scanning the QR code below to install the corresponding build.

	App Name	Jetpack
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr21663-03d19c5
	Commit	03d19c5
	Direct Download	jetpack-prototype-build-pr21663-03d19c5.apk

Note: Google Login is not supported on these builds.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WordPress by scanning the QR code below to install the corresponding build.

	App Name	WordPress
	Flavor	Jalapeno
	Build Type	Debug
	Version	pr21663-03d19c5
	Commit	03d19c5
	Direct Download	wordpress-prototype-build-pr21663-03d19c5.apk

Note: Google Login is not supported on these builds.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 39.42%. Comparing base (d3adbb6) to head (03d19c5).

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk   #21663   +/-   ##
=======================================
  Coverage   39.42%   39.42%           
=======================================
  Files        2122     2122           
  Lines       99558    99558           
  Branches    15317    15317           
=======================================
  Hits        39247    39247           
  Misses      56832    56832           
  Partials     3479     3479           

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