Fix issues #16655104 and #16875776 in battery stats.

Issue #16655104: Android system used 21% of battery

We weren't clearing the current process state when we now retain
process objects across resets.

Issue #16875776: abnormal outliers in screen-off battery life in go/batterystats

The checkin data since charged was using the since unplugged data
for the amount of drain with the screen on/off.

Also added a new version tag in the checkin output containing
the platform build number and internal version format number, to
start moving away from the wasteful version code on every line.

And fixed a bug in figuring out when things have changed over
discharge levels.

Change-Id: I89b89513a748016aacc848f14bbbc8b2400d92ca