For #15960 - Enable Grid View in Nightly only

[gabrielluong]

Fixes #15960

Pull Request checklist

• Tests: This PR includes thorough tests or an explanation of why it does not
• Screenshots: This PR includes screenshots or GIFs of the changes made or an explanation of why it does not
• Accessibility: The code in this PR follows accessibility best practices or does not include any user facing features. In addition, it includes a screenshot of a successful accessibility scan to ensure no new defects are added to the product.

To download an APK when reviewing a PR:

1. click on Show All Checks,
2. click Details next to "Taskcluster (pull_request)" after it appears and then finishes with a green checkmark,
3. click on the "Fenix - assemble" task, then click "Run Artifacts".
4. the APK links should be on the left side of the screen, named for each CPU architecture

[codecov-io]

Codecov Report

Merging #16462 (baa5605) into master (517ff91) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master   #16462   +/-   ##
=========================================
  Coverage     29.98%   29.98%           
  Complexity     1220     1220           
=========================================
  Files           455      455           
  Lines         18703    18703           
  Branches       2571     2571           
=========================================
  Hits           5609     5609           
  Misses        12659    12659           
  Partials        435      435           

Impacted Files	Coverage Δ	Complexity Δ
.../src/main/java/org/mozilla/fenix/utils/Settings.kt	70.90% <100.00%> (ø)	35.00 <0.00> (ø)

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