[Notifications P1] Add missing notification selection color for a split screen

[justtwago]

Fixes #22672

This PR fixes the lack of the notification rows' selection style on iPad (during split notifications mode). I also modified the rows' paddings to make the selection look natural. iPad multitasking mode is also handled to switch the selection style accordingly to the screen's width.

Before	After

Selection with no padding changes

To test:

• Run the app on an iPad
• Go to Notifications and ensure that a selected notification row has a grey-ish background
• Run the app on an iPhone
• Go to Notifications and make sure that by tapping on a notification, you can't see any row's background changes

Regression Notes

1. Potential unintended areas of impact
   Notifications screen

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

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

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• 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:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• 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)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

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	Build Number	pr22686-f0163e3
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[wpmobilebot]

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

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22686-f0163e3
	Version	24.3
	Bundle ID	com.jetpack.alpha
	Commit	f0163e3
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[alpavanoglu]

Is it possible to name this to something more elaborate? I couldn't understand what it is without reading its usage. Also could you move this enum to the top of the class right below the class signature (unless you're against it)? For public nested types, it is common to list them on the top so when a person views the file, all relevant types are at the top rather than being scattered through the file.

    enum ... {
        ...
    }

[justtwago]

Totally makes sense! Will try to find more accurate naming.

[alpavanoglu]

We are enabling it for all cases except for compact split view, correct?

[justtwago]

Yep. It's also disabled for the split mode on iPad:

Simulator.Screen.Recording.-.iPad.Pro.12.9-inch.6th.generation.-.2024-02-22.at.15.26.30.mp4

[alpavanoglu]

Thanks for the nice fix! I've also noticed that the content's position in the cell wasn't centered. Even if selectionStyle would be none, this is a nice improvement.

[alpavanoglu]

LG2M