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: Ie4026a7cc8aabb2a6d47d03d2e278aa51c2d5ddf