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

ui/build/util.go

@@ -15,6 +15,7 @@
 package build
 
 import (
+	"io"
 	"os"
 	"path/filepath"
 	"strings"
@@ -124,3 +125,20 @@ func decodeKeyValue(str string) (string, string, bool) {
 	}
 	return str[:idx], str[idx+1:], true
 }
+
+// copyFile copies a file from src to dst. filepath.Dir(dst) must exist.
+func copyFile(src, dst string) (int64, error) {
+	source, err := os.Open(src)
+	if err != nil {
+		return 0, err
+	}
+	defer source.Close()
+
+	destination, err := os.Create(dst)
+	if err != nil {
+		return 0, err
+	}
+	defer destination.Close()
+
+	return io.Copy(destination, source)
+}