Extract duplicated inDarkMode() helper to UIUtils

[aaronbrethorst]

Problem

inDarkMode() is now duplicated in two map fragment implementations:

• BaseMapFragment.java (Google Maps variant)
• MapLibreMapFragment.java (MapLibre variant)

If the dark mode detection logic needs to change (e.g., adding a user preference override), both would need updating independently.

Suggested Fix

Extract inDarkMode() to UIUtils (which already has theme-related helpers) as a static method, then call it from both map fragments.

Context

Noticed during review of #1532, which correctly copied the pattern from BaseMapFragment to MapLibreMapFragment.