Clean up of the print APIs.

1. Fixed a case in which the onPrintCompleted callback may not be invoked.

2. Added string resulrces for the different error messages for failed
   printing in OnPrintCompletedCallback

3. Fixed the string comments for the MediaSize constants.

4. Added APIs to create certain print attributes by passing in a
   string label instead of resource and package name since in some cases
   a print service may be getting already internationalized lables
   or it may not know all possible values.

5. Fixed a crash in PrintJobConfigActivity if there is no input or
   output tray.

Change-Id: Ie877b7dc669051606394aa290e6d2b979a42db4c