[Tablet] Reconsider design of Note Editor Previewer

* Ref: https://github.com/ankidroid/Anki-Android/pull/18724
* Ref: https://github.com/ankidroid/Anki-Android/pull/19988

2.24 introduced a split pane preview to the note editor. It also moved from `layout-xlarge` to `layout-sw600dp`.

A number of users have requested that a toggle is added: https://forums.ankiweb.net/t/uncomfortable-using-new-split-screen-in-editor/69656 (comments split across Discord, Play Store and Reddit).

By far the most convincing argument is by `phannan`:

> I appreciate all the hard work that went into doing the tabs, and unfortunately, it isn’t helpful for me. Especially in the edit screen, the old version works a lot better for me. I am lower vision, so everything is blown up. Losing a part of the screen makes both sides unusable. (See attached) Yes, I can flip to portrait mode, but then I lose half the card top to bottom. I’d appreciate having the option to turn this off, especially in the edit screen.
>
>

-- https://forums.ankiweb.net/t/uncomfortable-using-new-split-screen-in-editor/69656/3?u=david

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I would propose tackling this in 2 phases:

**2.24.X**
* Evaluate and reconsider `sw600dp` with real world examples
* Very likely: move from `sw` (smallest side width) to `w` (current width), and select a larger value
* Determine a justified value for foldables
* Potentially revert to `sw720dp` (roughly `layout-xlarge`)

**2.25.0** (expedited release for this change)
* Add a setting to Note Editor toggling the sidebar (and enabling the preview icon)