As per #10206: fix/improve LiveDevelopment init.

This is for #10206.

PreferencesManager.on("change") was executing before AppInit.appReady().
This was causing various inconsistencies, including setting up the UI before
LiveDevelopment is actually initialized. Handling of pref change is now moved
to AppInit.appReady and the implementation is set only after the modules are
actually initialized.

Minor nits included: pref definition improved, suite name made shorter, spare lines removed.

CC: @peterflynn, @sebaslv, @redmunds 
This actually should be `PrefixedPreferencesSystem`, I think.