BatteryStats: Record cpu power usage from /proc/uid_cputime

For now we are just recording the power usage and not using it
to calculate battery power usage or app blame. If it looks like
it is accurate, we'll adopt the values from the kernel instead of
estimating ourselves.

Bug:21498425
Change-Id: I6617e3c0ff279a65f4ff84472082f36fe4beb336