Print UI bug fixing and printer discovery refactoring.

1. Added support for selecting a printer from the all printers activity
   that is not in the initial printer selection drop down. The user
   initially sees a sub set of the printers in the drop down and the
   last option is to see all printers in a separate activity. Some
   of the printers in the all printers activity are not shown in the
   initial drop down.

2. Refactored printer discovery by adding (private for now) printer
   discovery app facing APIs. These APIs are needed to support multiple
   printer selection activities (print dialog and all printers activities)
   and also the settings for showing all printers for a service.

   Now multiple apps can request observing for printers and there is
   a centralized mediator that ensures the same printer discovery
   session is used. The mediator dispatches printer discovery specific
   requests to print services. It also aggregates discovered printers
   and delivers them to the interested apps. The mediator minimizes
   printer discovery session creation and starting and stopping discovery
   by sharing the same discovery session and discovery window with
   multiple apps. Lastly, the mediator takes care of print services
   enabled during discovery by bringing them up to the current
   discovery state (create discovery session and start discovery if
   needed). The mediator also reports disappearing of the printers
   of a service removed during discovery and notifies a newly
   registered observers for the currnet printers if the observers are
   added during an active printer discovery session.

3. Fixed bugs in the print UI and implemented some UX tweaks.

Change-Id: I4d0b0c5a6c6f1809b2ba5dbc8e9d63ab3d48f1ef

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