DO NOT MERGE Cherry-pick of Ie4026a7c back to GB

Original change description follows:
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Implement issue #3326435: Battery stats improvements

Keep track of discharge while screen is on vs. off.

Checkin looks like:

5,0,u,dc,1,1,1,0

The last four numbers are, from left:

- Maximum battery drain over time period.
- Minimum battery drain over time period.
- Battery drain while screen was on.
- Battery drain while screen was off.

Change-Id: Ie3cfe52df29b3f28ba8dc3350abe6cc967c76324