Update preference when displayed

If the preference is toggled on the subpage, hitting back to go
to the Display page may show the old setting value. Triggering
an update onStart will ensure the latest information.

NO_IFTTT=These changes are new and do not require backport

Fixes: 412273743
Test: manual - update AOD on subpage, go back to Display page
Flag: com.android.systemui.shared.ambient_aod
Change-Id: Ief950e5e71301ad898090042001e4f1d8128cae8