metrics: Enable for non-official builds.

The metrics uploader used to be disabled for non-official images to
avoid polluting the production data with possibly wrong measurements.
This is inconvenient for developers as they can only be sure that a new
metric was added properly until the changes reached a product device.

Instead, this CL change the metrics daemon to upload metrics iff the
consent file exists.
To ensure that testing data does not pollute the production data, we set
the channel to UNKNOWN and the version to 0.0.0.0 when the image is not
official (implied by channel and version missing).

BUG: 22879597

Change-Id: If03847090b732cc06270cbcc8b386b5f9e544a3d