add missing import in DownloadNotificationManager

app/src/fatweb/java/se.leap.bitmaskclient/appUpdate/DownloadNotificationManager.java

@@ -23,6 +23,7 @@ import android.app.NotificationManager;
 import android.app.PendingIntent;
 import android.content.Context;
 import android.content.Intent;
+import android.graphics.Color;
 import android.os.Build;
 
 import androidx.annotation.NonNull;