Doze Mode: ArcIdleManager metrics - activation time

Measure and report the time elapsed with the screen off due
to Doze Mode feature, and indirectly, the # of times, via
histogram counters.

Since we get the # of samples on a per-group basis, having the # of
samples be zero in the control group and non-zero in other groups will
confirm that the code is running as expected.  Contrasting the number of
activations and the time spent sleeping will allow us to make a decision
of where to prioritize optimizations: users on-battery vs users
on-AC-power.

TEST: out/Debug_chromeos/unit_tests --single-process-tests
    '--gtest_filter=ArcIdleManagerTest.*'

Bug: b:258072954
Change-Id: I12f864c516ec8e8477c4650052872668a5061de8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4530061


Reviewed-by: Yury Khmel <khmel@chromium.org>
Auto-Submit: Alexandre Marciano Gimenez <raging@google.com>
Reviewed-by: Youkichi Hosoi <youkichihosoi@chromium.org>
Commit-Queue: Alexandre Marciano Gimenez <raging@google.com>
Cr-Commit-Position: refs/heads/main@{#1146402}