Rewrite workflow on how the destination printer is selected.

This fixes two bugs
(1)
1. Printer was unknown when "all printer" activity was opened
2. Printer gets selected in "all printers" activity
3. Printer is unknown at the time the activity returns and thereby does
not get selected

(2)
If a print service gets disabled _all_ printers of the print service
will be marked as "removed" but do not removed from the adapter. This
is superflous as none of them can be selected anymore. The only thing
we want to make sure of is that the currently selected printer does
not get removed.

The workflow is now always:
- set mCurrentPrinter if needed
- Adjust mPrinterHolders
- adapter.notifyDataSetChanged()
  -> PrintersObserver.onChanged()
     -> onItemSelected

Bug: 27643305
Change-Id: I35ea7078367ad5a918c6362c04a26c4326e6c3f4