1. Factored out the printer discovery APIs of a print service in a
dedicated session object that is created by the print service on
demand. This ensures that added/removed/updated printers from
one session do not interfere with another session.
2. Updated the app facing APIs to pass in a document info along
with a printed file. Also exposed the print file adapter so
apps that create a temporary file for printing can intercept
when it is read by the system so the file can be deleted.
3. Updated the print service documentation.
Change-Id: I3473d586c26d8bda1cf7e2bdacb441aa9df982ed
798bed6cc7