Add passive_metrics USE flag to platform2

We want to be able to control the inclusion of the metrics_daemon,
which passively gathers power stats and the like and sends them
through the UMA pipeline iff the user has opted-in to metrics
collection.

CQ-DEPEND=CL:177244, CL:177291
BUG=None
TEST=emerge platform2 with USE='-passive_metrics' on and off, with and without FEATURES=test

Change-Id: Id6af84d3de585a156d8ebbcc23117df000de080d
Reviewed-on: https://chromium-review.googlesource.com/177217


Reviewed-by: Chris Masone <cmasone@chromium.org>
Tested-by: Chris Masone <cmasone@chromium.org>
Commit-Queue: Chris Masone <cmasone@chromium.org>