Run the metrics uploader in the background.

The metrics uploader was currently running on foreground where it
would copy the metrics files in a separate directory and then forked
into the background for the upload process. As a result, running the
lunch command would take a second longer to run since each metrics
uploader run had an average of half a second.

Bug: 140638454
Test: * Wrote and updated unit test cases.
      * Set ANDROID_ENABLE_METRICS_UPLOAD to point to the latest
        metrics_uploader bash script. Executed the "lunch 1" command
	and measured the running time. Executed "m nothing" command
	and checked that the metrics were uploaded.
      * Ran "lunch 1" and "m nothing" with
        ANDROID_ENABLE_METRICS_UPLOAD=""
      * Removed oauth from metrics_uploader and ran "m nothing" and
        "lunch 1". The oauth Message appeared only to "m nothing"

Change-Id: I13c61e666c8f44613dee291a704cef6a27335188