Add RBE metrics dump in Soong UI.

From aosp/1329396, the RBE metrics protobuf file is part of the
metrics uploading process. The RBE metrics protobuf file is
generated by running the bootstrap shutdown command. A new function
named DumpRBEMetrics was written in order to generate the RBE metrics
protobuf file before sending to the uploading process.

Bug: b/140638454
Test: * Unit test cases
      * Ran RBE build on my local host and verified the
        metrics protobuf file is created.
      * Ran non-RBE build after RBE build and verified that
        the previous metrics protobuf file was deleted.

Change-Id: I4b8068905cb67c4b8c2d94793917b98974fed707
diff --git a/ui/build/Android.bp b/ui/build/Android.bp
index 0a0bb16..4ef2721 100644
--- a/ui/build/Android.bp
+++ b/ui/build/Android.bp
@@ -63,6 +63,7 @@
         "cleanbuild_test.go",
         "config_test.go",
         "environment_test.go",
+        "rbe_test.go",
         "upload_test.go",
         "util_test.go",
         "proc_sync_test.go",