Add JavaScript method to get the system locale

app/src/main/java/org/mozilla/webmaker/javascript/WebAppInterface.java

@@ -22,6 +22,8 @@ import org.mozilla.webmaker.activity.Element;
 import org.mozilla.webmaker.router.Router;
 import org.mozilla.webmaker.storage.MemStorage;
 
+import java.util.Locale;
+
 public class WebAppInterface {
 
     protected Context mContext;
@@ -290,4 +292,14 @@ public class WebAppInterface {
     public boolean isDebugBuild() {
         return BuildConfig.DEBUG;
     }
+
+    /**
+     * ----------------------------------------
+     * Get System Locale
+     * ----------------------------------------
+     */
+    @JavascriptInterface
+    public String getSystemLocale() {
+        return Locale.getDefault().toString();
+    }
 }