ActiveDisplay: reduce some IO performed in regularly accessed methods

Previously inQuietHours() method was run nearly every time ActiveDisplay did anything.
Since it's optional if quiet hours settings will be observed we only need to run this method
when the user has chosen this behavior and futher we only need to do IO for the quiet hours
settings if quiet hours is enabled.

PS2: initialize uninitialzed variable mQuietTime and add missing return statement

Change-Id: I964fa0b56ef4296b6e9bf5c11d48c65a105e61eb