Coalescing multiple print job notifications.

1. We used to show a single notificaiton for every print job but
   this is against th UX guidelines. Since we have to lead by
   example, this change adds coalescing of multiple notifications.

bug:11155212

2. Print job state callback in the PrintManager now correctly
   invoked on the main thread.

bug:10983508

Change-Id: I906e9e62198fa37cb4d2f71fce07ed475d61e1bd