Track foreground activities in battery stats.

To help correlate battery usage against actual foreground app usage,
start tracking time on a per UID basis.  It uses the nesting feature
of StopwatchTimer to track multiple PIDs inside the same UID.

Improves units on docs, and persists foreground CPU time with a
consistent ordering.  Reports foreground activities time and
foreground CPU time.

Change-Id: I95d12963923e3fd421730f6fbfc842dfd8d3a055