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/Android.bp b/ui/build/Android.bp
index 1ddaf68..6f81c17 100644
--- a/ui/build/Android.bp
+++ b/ui/build/Android.bp
@@ -52,6 +52,7 @@
         "ninja.go",
         "path.go",
         "proc_sync.go",
+        "rbe.go",
         "signal.go",
         "soong.go",
         "test_build.go",