Attribute UNKNOWN display state to OFF

When device restarts, screen state is set to unknown, all
timebases stop. Then if setBatteryStateLocked() is called
with unplugged state, onBatteryTimeBase starts, but
onBatteryScreenOffTimeBase does not, so does ScreenOnTimer,
since screen state == unknown. So this duration is lost.
This fix attributes unknown state to ScreenOff.

Fixes: 67608353
Test: added a unit test in BatteryStatsNoteTest
Change-Id: Iee68898522564963e7d0b0110f1aee9248aacfe8