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