Validate QuestionnaireItem.Group with repeats as repeating group

[LZRS]

Instead of just as group

IMPORTANT: All PRs must be linked to an issue (except for extremely trivial and straightforward changes).

Fixes #2752

Description
Clear and concise code change description.

Alternative(s) considered
Have you considered any alternatives? And if so, why have you chosen the approach in this PR?

Type
Choose one: (Bug fix | Feature | Documentation | Testing | Code health | Builds | Releases | Other)

Screenshots (if applicable)

Checklist

• I have read and acknowledged the Code of conduct.
• I have read the Contributing page.
• I have signed the Google Individual CLA, or I am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have run ./gradlew spotlessApply and ./gradlew spotlessCheck to check my code follows the style guide of this project.
• I have run ./gradlew check and ./gradlew connectedCheck to test my changes locally.
• I have built and run the demo app(s) to verify my change fixes the issue and/or does not break the demo app(s).

[jingtang10]

code looks good but please add tests