Add tests for DeviceInfoModule.getWindowDisplayMetrics edge-to-edge branches

[zoontek]

Summary:

This PR adds two focused unit tests for DeviceInfoModule.getWindowDisplayMetrics() that attach a mocked Activity, swap in a fake WindowMetricsCalculator via WindowMetricsCalculator.overrideDecorator() and verify both branches of isEdgeToEdgeFeatureFlagOn:

• edge-to-edge on -> WindowMetrics.bounds are used directly.
• edge-to-edge off -> bounds minus systemBars | displayCutout insets.

To make this testable without reflection or @SuppressLint("RestrictedApi") outside the production code, three small visibility tweaks:

• DeviceInfoModule.getWindowDisplayMetrics() and getDisplayMetricsWritableMap(): private → internal + @VisibleForTesting
• WindowUtilKt.isEdgeToEdgeFeatureFlagOn setter: private set → @VisibleForTesting internal set

Changelog:

[INTERNAL] [ADDED] - Add unit tests for DeviceInfoModule.getWindowDisplayMetrics covering both edge-to-edge branches

Test Plan:

./gradlew :packages:react-native:ReactAndroid:testDebugUnitTest \
  --tests com.facebook.react.modules.deviceinfo.DeviceInfoModuleTest