Enable highlighting of Print services

- add to SettingsPreferenceFragment to know when the associated
RootAdapter is changing and thus being able to find the highlighted
Preference
- also, add the correct Search key during indexing. She should
match the one created when adding dynamically the Preference
- last, increase to 400ms the delay to do the highlight

Change-Id: I3a1a81fdf5c8ab5f3aaab29f16ea9879ab6df056