metrics: Disable uploader on non-official build.

The metrics uploader should only be enabled on an official build.

BUG=chromium:459105
TEST=`FEATURES=test emerge-$BOARD metrics`
TEST=Verified that metrics uploader is disabled on a developer build.

Change-Id: I5cadb3afeb56c0adac971228aa48ad56ed913bbf
Reviewed-on: https://chromium-review.googlesource.com/250542


Reviewed-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>