Defers initialization of preferences.

In Screen Timeout Settings, preferences should not be initialized in
onAttach() because the setting style hasn't been loaded yet. This change
defers the initialiaztion of those preferences so that they appear
correctly.

Test: manually tested.
Test: make RunSettingsRoboTests ROBOTEST_FILTER=com.android.settings.display
Bug: 183909540

Change-Id: I86cfe196549d709ed763faa004fff7b631365b1e