indicatorColor: use primary for both light and dark themes (#309)

It's only used for M2 tabs in Flutter but make it consistent between the
light and dark themes anyway so it'll be possible to share the theme
creation without parameterizing indicatorColor.

lib/src/themes/common_themes.dart

@@ -417,7 +417,7 @@ ThemeData createYaruLightTheme({
     dividerColor: kDividerColorLight,
     dialogBackgroundColor: colorScheme.background,
     textTheme: createTextTheme(YaruColors.inkstone),
-    indicatorColor: colorScheme.secondary,
+    indicatorColor: colorScheme.primary,
     applyElevationOverlayColor: false,
     buttonTheme: _buttonThemeData,
     outlinedButtonTheme: _createOutlinedButtonThemeData(colorScheme),