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
3e013e85b0