Change "system default" string for consistency

Use the same string as the Android language selector

app/src/main/res/values/strings.xml

@@ -118,7 +118,7 @@
     <string name="other_prefs">Other</string>
     <string name="app_language">App language</string>
     <string name="lang_english">English</string>
-    <string name="system_default">System</string>
+    <string name="system_default">System default</string>
     <string name="no_activity_file_selection">No app found to handle file selection</string>
     <string name="file_saved_with_name">File saved as \"%1$s\"</string>
     <string name="theme_light">Light</string>