Fix Magic Number problem in DownloadFile (caused by #431).

ultrasonic/src/main/kotlin/org/moire/ultrasonic/service/DownloadFile.kt

@@ -45,7 +45,7 @@ class DownloadFile(
     private val mediaStoreService: MediaStoreService
     private var downloadTask: CancellableTask? = null
     var isFailed = false
-    private var retryCount = 5
+    private var retryCount = MAX_RETRIES
 
     private val desiredBitRate: Int = Util.getMaxBitRate(context)
 
@@ -382,4 +382,8 @@ class DownloadFile(
             }
         }
     }
+
+    companion object {
+        const val MAX_RETRIES = 5
+    }
 }