Small print API tweaks

1. Removed an unused public constant

2. Hide PrintFileDocumentAdapter which makes doing the wring
   thing easy.

3. We allow a print service to set a tag of a print job which is
   an arbitrary string it only knows how to interpret. Typically,
   this is the id of a remote print job. This tag was visible to
   applications which is wrong - this is data that should be
   private to the print service. Now the print service is the
   only one to see that.

bug:11084635

Change-Id: I763ea9ff0fdf647805bc36e1737d72263090714d