[Android] Implement W3C WebNotification API

W3C web notification API is defined at http://www.w3.org/TR/notifications/.
On Android, web notifications are well matched to system notifications.
To invoke js callbacks when system notification got clicked or canceled,
two pendingIntent is binded to the notification.
Since Android only keeps one pendingIntent for specified Context/Class/Category,
the unique id of the notification is used as the Category of the pendingIntents.

TEST=XWalkCoreTest org.xwalk.core.xwview.test.WebNotificationTest
BUG=https://crosswalk-project.org/jira/browse/XWALK-94