metricsd: Collect average cpu usage information.

Collect a new histogram (Platform.CpuUsage.Percent), collecting the
average cpu usage in percent of the total cpu usage possible (across all
cpus).
This measurement is collected every minute.

Bug: 25231576
Test: unit tests.
Test: Manual testing.

Change-Id: I1c63486c177f0c2d4bd361eb9e351a7ca25b80d9
6 files changed