[Android] Customize the resource loading of MediaPlayer

Add XWalkMediaPlayerResourceLoadingFilter which inherits
MediaPlayerBridge.ResourceLoadingFilter. Then we can customize
the MediaPlayer in xwalk. Now we have two customized items:
    Support app scheme in the resource loading of MediaPlayer.
    Support file://android_asset/ url in MediaPlayer.

BUG=https://crosswalk-project.org/jira/browse/XWALK-880
Merge pull request #1539 from xingnan/mediaplayer

[Android] Customize the resource loading of MediaPlayer
Override notification related interface from content

There are four methods in content browser client which are for
client to implement to support web notification.
Override those apis with empty function bodies first.
[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 pendingIntents are 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.

Platforms other than Android keeps empty implementaion for notification related
interfaces.

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