Missed signal in FusedPrintersLoader.

1. In the FusedPrintersLoader we start observing the printers on
   the device and if they change we send the result. If however,
   the printers are already loaded in our session (because it
   joined an ongoing printer discovery) and the pritners do not
   change, the loader never sends its result. Now we are registring
   the callback only after historical printers are loaded. We
   also immediately check after starting discovery whether the
   there are printers in the discovery session and if so deliver
   them.

2. Improved logging in the FusedPrintersLoader.

bug:10940712

Change-Id: Ieb9b897d64780742125b29309462dea3eda170a6