metricsd: Read build time values from etc/os-release.d.

This stops relying on system properties to provide build time
configuration.
Product version and id will be stored in /etc/os-release.d.
Channel will be pulled from update engine.

BUG: 24947119
Change-Id: I0972d03cd83ef622846de3cce3dec1992fcc46cd
6 files changed