[Android] Unregister broadcast receiver for notification properly

Unregister non-regsitered broadcast receiver will cause
crash on Android. The condition to detect whether the broadcast
receiver has been registered to receive Notification close
pending intent is change by commit eeeb8eeab.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2356
Merge pull request #2326 from wang16/notification

[Android] Unregister broadcast receiver for notification properly