Don't change screen on time on time changes

Screen on time should be measured in elapsed realtime, not wallclock.

Cause a checkIdleStates to occur when reloading stats
(on rollover and on time change).

When time changes occur in the negative direction, the new stats file
we create can overlap the previous one with regards to its end timestamp.
Use the begin timestamp to determine which of the latest stats to merge.
(b/22716352)

Bug: 26488100
Change-Id: If31b29bbbee9e98401205b5e26bce86e181286e7