Refinement of the print service APIs.

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