Set ANDROID_ENABLE_METRICS_UPLOAD to the metrics_uploader path to upload metrics.

This is to collect metrics from our RBE dogfooders. This CL will be
rolledback once the metrics pipeline is turned live.

Bug: b/140638454
Test: source the script and then "m nothing". Monitored the uploading
      of the metrics.
Change-Id: Ide996a44a5a651ed026d7c8272283675fbb092b5