Better handling of PublishFeedDialog result

Using FacebookDialog.getNativeDialogCompletionGesture as the indicator for better handling, as postId will return null without [publish_actions] permission granted.

Ref:
https://developers.facebook.com/docs/reference/android/current/class/FacebookDialog/
Update PublishFeedDialogAction.java
Merge pull request #238 from booker0108/patch-1

Better handling of PublishFeedDialog result