Print API updated.

1. Removed the updatePrinters API on PrinterDiscoverySession.
   Now re-adding a printer updates it.

2. Added getTrackedPrinters() API to allow a print service to
   figure out which printers' state should be tracked by the
   service.

3. Removed the APIs on PrintDocumentInfo that describe the
   layout of the content as they are not needed. For example,
   if the print attributes passed in layout require landscape
   mode, the the app should just create a PDF page with the
   wider side up and draw its content in portrait. The printer
   will then rotate the page.

Change-Id: Idd72c6e9c129e8b17eef1236573a99773f8ff0a2