fix bug in cache cleanup on 21+ introduced in cbd6da5

!379 was merged without the emulator tests passing.  This fixes that.

app/src/main/java/org/fdroid/fdroid/CleanCacheService.java

@@ -167,7 +167,7 @@ public class CleanCacheService extends IntentService {
                 f.delete();
             }
         } else {
-            CleanCacheService21.deleteIfOld(f, millisAgo);
+            CleanCacheService21.deleteIfOld(f, olderThan);
         }
     }
 }