Add a condition timer to track the duration of condition being true.

+ Plug it in ValueMetric as the frist step as we often need to normalize the values
  by the duration of condition being true.

  E.g., Coulomb counter drop / screen_on_and_on_battery_time

  Having a simple timer inside the metric will eliminate the need of creating
  another duration metric and joining them on server side.

Test: statsd_test
Fix: 130048020

Change-Id: Ie377dc6de917176ab07843e0dfd8506ed1d5db75