refactor: use field backing property in Instant Editor VM

[criticalAY]

Purpose / Description

Use the new field backing property in the ViewModels in our codebase

Fixes

NA

Approach

Replace the existing code _somethingwith field and update the usage
The commits are clear as to what VM I touched and what I changed

How Has This Been Tested?

Local compile and tested the changes on my Pixel

Learning (optional, can help others)

NA

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[david-allison]

Might have wanted a [delegated] property on currentClozeNumber, but seems OK to me