Add USE_RBE support to soong.

Test: Built aosp_arm-user with and without USE_RBE. USE_RBE uses
a proxy script in place of rewrapper.

Change-Id: I5bf008a940513872d70b5b215bd6209f759826ae
diff --git a/ui/build/ninja.go b/ui/build/ninja.go
index b41ac20..66750d6 100644
--- a/ui/build/ninja.go
+++ b/ui/build/ninja.go
@@ -43,7 +43,7 @@
 	args = append(args, config.NinjaArgs()...)
 
 	var parallel int
-	if config.UseGoma() {
+	if config.UseGoma() || config.UseRBE() {
 		parallel = config.RemoteParallel()
 	} else {
 		parallel = config.Parallel()