Fix part of #5344: Add new feature flag for multiple classrooms

[theMr17]

Explanation

Fixes part of #5344

This PR introduces a feature flag ENABLE_MULTIPLE_CLASSROOMS which will be used to gate the new multiple classrooms screens.

Essential Checklist

• The PR title and explanation each start with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
• The PR does not contain any unnecessary code changes from Android Studio (reference).
• The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
• The PR is assigned to the appropriate reviewers (reference).

For UI-specific PRs only

If your PR includes UI-related changes, then:

• Add screenshots for portrait/landscape for both a tablet & phone of the before & after UI changes
• For the screenshots above, include both English and pseudo-localized (RTL) screenshots (see RTL guide)
• Add a video showing the full UX flow with a screen reader enabled (see accessibility guide)
• For PRs introducing new UI elements or color changes, both light and dark mode screenshots must be included
• Add a screenshot demonstrating that you ran affected Espresso tests locally & that they're passing

[theMr17]

@adhiamboperes PTAL.

[adhiamboperes]

LGTM, thanks @theMr17!

[oppiabot]

Assigning @BenHenning for code owner reviews. Thanks!

[BenHenning]

Thanks @theMr17! This LGTM.

[BenHenning]

Enabling auto-merge since everything looks good for this PR.

[oppiabot]

Hi @theMr17, this PR is ready to be merged. Please address any remaining comments prior to merging, and feel free to merge this PR once the CI checks pass and you're happy with it. Thanks!