perfboot: significant figures + units for summary

This is correct significant figures/units based on the precision of our
measurements, but it does not reflect our actual certainty re the output
data, since in reality, confidence is diminished by temperature, device,
hardware revision, time of day/month/year, spurious activity, data
connectivity, app install list, inherent randomness of multiprogramming,
sensor activity, user interaction, /data caches, build-by-build
differences, charging state, data fragmentation, race-driven sleeps,
cosmic radiation, factory defects, local magnetic or gravitational field
differences, changes in device momentum, or other known and unknown
causes.

Bug: N/A
Test: run perfboot.py, and output has:
    mean: 10801 ms
    median: 10801 ms
    standard deviation: 18 ms
Change-Id: I796396acc203b29e9a14e4d6dffa58db7b8cd9fb