metrics: add support for other product id

metrics_uploader should use the GOOGLE_METRICS_PRODUCT_ID field from os-release
whenever possible instead of the default Chrome product id.

BUG=chromium:415744
TEST=FEATURES=test emerge-gizmo metrics succeeds.
TEST=test_that platform_MetricsUploader succeeds.

CQ-DEPEND=CL:221963

Change-Id: I69b1a6ca766048ad80d93008a2fe3b18879bf1da
Reviewed-on: https://chromium-review.googlesource.com/221953


Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>