Add Heads Up notification display à la L-preview

This is based on the implementation done by SlimRoms.
Heads Up notifications only show up for high priority notifications.
Heads Up notifications can be snoozed.
Heads Up notifications can be displayed at the bottom of the screen.
Heads Up notifications are disabled by default and need to be enabled per app.
To enable Heads Up notifications for an app,
1. Go to Settings > Apps > Manage Apps
2. Select the app
3. Select "Show notifications" if it is deselected
4. Select "Enable heads up notification"

https://gerrit.slimroms.net/#/c/9942/
https://gerrit.slimroms.net/#/c/12104/