Marshal Metrics

Because DRM Plugins will not be allowed to call the AMI API under
Treble, the mediadrmservice will need to marshal the metrics out of the
plugin in protobuf format and report them to the AMI API instead. This
patch implements the pulling and interpreting of metrics from DRM
Plugins.

Bug: 36497276
Test: Played Google Play Movies, verified that playback still worked and
      Widevine metrics appeared in a dump of the system media metrics.
Change-Id: If07717c1b87022bc1fcdedfbc62b9193899742d5