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As it turns out, the per-core time_in_state data from
/sys/devices/system/cpu/cpuX/cpufreq/stats/time_in_state is wall time
rather than CPU time and includes time the cores spend in idle states.

Consequently, the data we currently report into Power.Approx* metrics
isn't correct. We have to correct for the idle time when computing each
core type + frequency state's proportional part of the execution.

This patch adds a way to do so by polling per-core idle times from
/sys/devices/system/cpu/cpuX/cpuidle/stateY/time (via a helper in
base::CPU) and updates the CpuTimeMetrics code to account for idle time.

Bug: 1081760
Change-Id: I7b8168b47b1e4d118bc7569aebb07952fa78693e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462088
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816509}