Add "add service" pref to the autofill picker

Mimicing the behavior we have for adding a print service.

Fixes: 37735998
Test: - Started auto fill picker with add-service-settings set and unset.
        Only if set, the preference is added
      - RunSettingsRoboTests (unrelated failure in
                              SetupChooseLockPatternTest)
      - Enable + disable auto fill service while settings is open
Change-Id: Ie019a032a1d01373f8fd0ebd7ae2ddf9c4d8871f