chore(mobile): reduce spacing on video controls (#27313)

The spacing was required for the old slider, but the new one has its own
spacing and makes it redundant. There is too much now, and we've
received feedback that it should be less sparse. The default track
height of 16px is an improvement over the old track height, but it is
very thick. A middleground of 12px might be better.

mobile/lib/theme/theme_data.dart

@@ -62,6 +62,7 @@ ThemeData getThemeData({required ColorScheme colorScheme, required Locale locale
     ),
     chipTheme: const ChipThemeData(side: BorderSide.none),
     sliderTheme: const SliderThemeData(
+      trackHeight: 12,
       // ignore: deprecated_member_use
       year2023: false,
     ),