[Reader] Scroll post list to top when Reader tab is tapped

[RenanLukas]

Fixes #15175

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To Test:

1. Install JP and sign in.
2. Open Reader.
3. 🔍 Open different feeds, scroll down to any point and tap the "Reader" tab in bottom navigation: the list should be scrolled to top and AppBar should be expanded.

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Regression Notes

1. Potential unintended areas of impact

   • None

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

   • Manual testing

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

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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.

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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 Warning
⚠️	PR is not assigned to a milestone.

Generated by 🚫 Danger

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[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	pr20544-00e8f7f
	Commit	00e8f7f
	Direct Download	wordpress-prototype-build-pr20544-00e8f7f.apk

Note: Google Login is not supported on these builds.

[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	pr20544-00e8f7f
	Commit	00e8f7f
	Direct Download	jetpack-prototype-build-pr20544-00e8f7f.apk

Note: Google Login is not supported on these builds.

[daniloercoli]

Great job! I'm like this feature a lot.

Should we implement the scroll to top functionality on other screens as well?
cc @osullivanchris
Specifically, the Notifications screen, as users often scroll to the bottom of it. (Maybe implement it on My Site as well the other 2 screens?).

[osullivanchris]

per our hangout I'm keen to have this on other tabs as well.

FYI this also came up within comments (which may now be at the bottom of a reader view in future) - which is slightly trickier to solve, as its not at the root level so can't solve by tapping the bottom tab bar.

I was playing around with this today where the title of the post would collapse into the navigation bar, and be tappable to scroll to the top. I don't love it yet though.

I also tried with a bit of UI at the bottom of the screen but it was a bit heavy.