Multiple printer discovery session instances and other bugs.

1. The fused printers provider was dropping on the floor received printers
    if it was not active. It is in fact a loaded and if not active it should compute
    the printers and not deliver them until activated. This fixes an issue where
    opening the print dialog, then enabling a print service results in the printers
    reported by the service not showing up in the print dialog.

2. Printer discovery session was created twice which leads to incorrect behavior
    as the pint system is designed around the contract that there is a single
    printer discovery session per service at a time. This was possible due to an
    incorrect initialization of a member variable resulting in double session creation
    when the print service is connected.

3. When a print service is enabled during discovery we did not use the correct
    condition to start printer discovery resulting in starting it all the time even if
    not needed. Also if some of the printers that had to be tracked are reported
    by the service just enabled (typically historical printers) we did not ask the
    service to start tracking them.

4. Removed some logging.

bug:10903343

Change-Id: I46c049471a4b099fc668df3aee2aaedc8d7786ac