Cleanup several warnings inside DisplayMetricsHolder (#51074)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/51074

While working on this class I noticed there are a number of warnings. Let's clean them up.

Changelog:
[Internal] [Changed] -

Reviewed By: mdvacca

Differential Revision: D74001269

fbshipit-source-id: 7e86ffdaf9c82ff5a165618813fa700f8e850a74