metrics: Move opt in into send functions

Refactored send_hits into metrics_util, and put the metrics opt-in
check into the send library functions so that code can just send
metrics without having to manually check it themselves.  Also
removed unused send_build.py script.

Bug: 27527684
Bug: 27759379
Test: `python test_runner.py` passes
Test: Removed brillo config.db file, verified commands prompt for
      metrics opt in

Change-Id: Id9eeb08e01bf08ded65124375af2d2d3f91e0890
9 files changed