base

Refactor ProcessStats, ProcessTracker.

ProcessStats is now called ProcessCpuTracker.

ProcessTracker is now ProcessStatsService, and its inner State
class is broken out into a separate top-level ProcessStats class.
This ProcessStats is moved to the framework, so we will be able
to use it elsewhere.

Change-Id: I6a127bcb835b6b474b72647c0b99b82c2137e5c5