Fix lint warning about string

android/java/app/src/main/res/layout/activity_main.xml

@@ -36,7 +36,7 @@
             android:layout_height="wrap_content"
             android:layout_gravity="right|top"
             android:padding="10dp"
-            android:contentDescription="Map compass. Click to reset the map rotation to North."
+            android:contentDescription="@string/compass_desc"
             android:src="@drawable/compass"/>
 
         <TextView

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

@@ -9,6 +9,8 @@
 
     <string name="label_fps">FPS:</string>
 
+    <string name="compass_desc">Map compass. Click to reset the map rotation to North.</string>
+
     <string-array name="style_list">
         <item>Bright</item>
         <item>Basic</item>