Use debug mode when running R8 for eng builds.

Bug: 119601874
Test: Built for different variants.
Change-Id: I2250587eb3e457ca15265eb9f8359872e9b08363
diff --git a/android/config.go b/android/config.go
index abb07ce..54c9da8 100644
--- a/android/config.go
+++ b/android/config.go
@@ -581,6 +581,10 @@
 	return Bool(c.productVariables.Debuggable)
 }
 
+func (c *config) Eng() bool {
+	return Bool(c.productVariables.Eng)
+}
+
 func (c *config) DevicePrefer32BitApps() bool {
 	return Bool(c.productVariables.DevicePrefer32BitApps)
 }