Use sampling for deviceutilstatsmonitor

Current device utilization mechanism seems flawed, with machines often
going into state where they report questionable metrics due to 'state end
already reported for device' errors.

This commit changes the util calculation to a simpler moving average mechanism.

Bug: 16016815

Change-Id: Ic86972479b79c319c829b78852103b65c71a5d91
6 files changed