[Tags Feed] Fix duplicate tracking in Reader subfilter

[thomashorta]

Fixes #20814

• Create mechanism to avoid multiple observers attached to the same observables
  • Making sure it works with Fragment system-initiated re-creation (e.g.: screen rotation)
  • Making sure it works with ViewModel cleared due to Owner destruction and creation with existing previous key
• Fix sending the reader_filter_sheet_item_selected when selecting a filterable feed (e.g.: Subscriptions and Your Tags)

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To Test:

1. Install Jetpack
2. Make sure App Setting -> Privacy -> Collect Information is enabled
3. Make sure reader_tags_feed Feature config is enabled (Debug Settings -> Remote Features
4. Go to Reader
5. Go to Subscriptions
6. Verify reader_filter_sheet_item_selected is not tracked
7. Tap the subfilter pill
8. Verify appropriate events are tracked ONLY ONCE
9. Tap a Site
10. Verify reader_filter_sheet_item_selected is tracked ONLY ONCE
11. Tap the pill again
12. Repeat 9-11 multiple times making sure events are tracked ONLY ONCE per trigger
13. Go to Your Tags
14. Repeat 6-12

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Regression Notes

1. Potential unintended areas of impact

   • N/A

2. What I did to test those areas of impact (or what existing automated tests I relied on)

   • N/A

3. What automated tests I added (or what prevented me from doing so)

   • Unit tests not added for the one-time initialization mechanism since it is done completely in Fragment and ViewModel non-exposed APIs (onCleared)
   • Unit tests updated for bug fix related to sending reader_filter_sheet_item_selected when opening a filterable feed

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PR Submission Checklist:

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

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

• [ ] WordPress.com sites and self-hosted Jetpack sites.
• [x] 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)