Add feature flag for gating NPS Survey

[adhiamboperes]

Explanation

Add feature flag for gating NPS Survey

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)
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• Add a screenshot demonstrating that you ran affected Espresso tests locally & that they're passing

[BenHenning]

Thanks @adhiamboperes! Just had one thought--PTAL.

[BenHenning]

Thanks @adhiamboperes! This LGTM.

[oppiabot]

Hi @adhiamboperes, 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!