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
diff --git a/ui/metrics/upload_proto/upload.proto b/ui/metrics/upload_proto/upload.proto
index 7a9f080..bcd0ab2 100644
--- a/ui/metrics/upload_proto/upload.proto
+++ b/ui/metrics/upload_proto/upload.proto
@@ -32,4 +32,8 @@
 
   // A list of metrics filepaths to upload.
   repeated string metrics_files = 5;
+
+  // A list of directories to delete after the copy of metrics files
+  // is completed for uploading.
+  repeated string directories_to_delete = 6;
 }