Safeguard findUniqueFilename from race conditions

This might partially, but not fully, solve Issue commons-app#228 (accidental overwriting).

Author: whym

app/src/main/java/fr/free/nrw/commons/upload/UploadService.java

@@ -284,7 +284,7 @@ private void showFailedNotification(Contribution contribution) {
         contribution.save();
     }
 
-    private String findUniqueFilename(String fileName) throws IOException {
+    synchronized private String findUniqueFilename(String fileName) throws IOException {
         return findUniqueFilename(fileName, 1);
     }