Fix flicker in Dark theme

Currently, when schedule sets to "Turns on at bedtime", the footer will
show a slid up animation when entering the page, this is because the
"Start time" & "End time" preferences are hidden in onResume().

This is because these 2 preferences always return AVAILABLE in
getAvailabilityStatus(), and manually update visibility in
refreshSummary(), which is called each time updateState() is called.

Usually the controller not set the visibility explicitly, but return
CONDITIONALLY_UNAVAILABLE in getAvailabilityStatus() when they want to
hide the preference.

Because getAvailabilityStatus() is called in onCreate(), by using this,
we can fix the flicker.

Fix: 234399017
Test: visual & robo test
Change-Id: I4cb7dd95d2985bd1ca4c8cb30aaebdc21a5415f8