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/android/config.go b/android/config.go
index 8ced93a..556ddcc 100644
--- a/android/config.go
+++ b/android/config.go
@@ -748,6 +748,10 @@
 	return Bool(c.productVariables.UseGoma)
 }
 
+func (c *config) UseRBE() bool {
+	return Bool(c.productVariables.UseRBE)
+}
+
 func (c *config) RunErrorProne() bool {
 	return c.IsEnvTrue("RUN_ERROR_PRONE")
 }