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/variable.go b/android/variable.go
index 47586a7..e9379b7 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -206,6 +206,7 @@
 	HostStaticBinaries               *bool `json:",omitempty"`
 	Binder32bit                      *bool `json:",omitempty"`
 	UseGoma                          *bool `json:",omitempty"`
+	UseRBE                           *bool `json:",omitempty"`
 	Debuggable                       *bool `json:",omitempty"`
 	Eng                              *bool `json:",omitempty"`
 	Treble_linker_namespaces         *bool `json:",omitempty"`