refactor(custom-study): use userInputValue

[david-allison]

A refactoring I noticed while doing #11116

Anki-Android/AnkiDroid/src/main/java/com/ichi2/anki/dialogs/customstudy/CustomStudyDialog.kt

Lines 142 to 143 in 4e2b20d

	private val userInputValue: Int?
	get() = binding.detailsEditText2.textAsIntOrNull()

Checklist

• 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
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• UI Changes: You have tested your change using the Google Accessibility Scanner