Fix android stat bug when stat asset file

src/android/src/main/java/com/RNFetchBlob/RNFetchBlobFS.java

      */
     static void stat(String path, Callback callback) {
         try {
-            callback.invoke(null, statFile(path));
+            WritableMap result = statFile(path);
+            if(result == null)
+                callback.invoke("stat error: failed to list path `" + path + "` for it is not exist or it is not a folder", null);
+            else
+                callback.invoke(null, result);
         } catch(Exception err) {
             callback.invoke(err.getLocalizedMessage());
         }

src/android/src/main/java/com/RNFetchBlob/RNFetchBlobReq.java

                     String filename = map.getString("filename");
                     // upload from storage
                     if(data.startsWith(filePathPrefix)) {
-                        File file = new File(data.substring(filePathPrefix.length()));
+                        String orgPath = data.substring(filePathPrefix.length());
+                        File file = new File(RNFetchBlobFS.normalizePath(orgPath));
                         form.addBinaryBody(name, file, ContentType.APPLICATION_OCTET_STREAM, filename);
                     }
                     // base64 embedded file content
             // upload from storage
             if(body.startsWith(filePathPrefix)) {
                 String filePath = body.substring(filePathPrefix.length());
-                entity = new FileEntity(new File(filePath));
+                entity = new FileEntity(new File(RNFetchBlobFS.normalizePath(filePath)));
             }
             else {
                 blob = Base64.decode(body, 0);