AppIdleHistory: Only write screen on durations during regular sync

Stop writing to disk every time the display goes off. Only write to
disk periodically.

Consequences

Previously, if the device rebooted after the screen duration was written
to disk, apps would appear more stale than they actually were.

Now apps will always look fresher, which is a better scenario.

Bug:30807864
Change-Id: Ia69a2e51fc9e397789215b449fae56fa3e29c74a