Use debug mode when running R8 for eng builds.

Bug: 119601874
Test: Built for different variants.
Change-Id: I2250587eb3e457ca15265eb9f8359872e9b08363
diff --git a/java/dex.go b/java/dex.go
index 5cec325..a6d486a 100644
--- a/java/dex.go
+++ b/java/dex.go
@@ -160,6 +160,11 @@
 	// TODO(ccross): if this is an instrumentation test of an obfuscated app, use the
 	// dictionary of the app and move the app from libraryjars to injars.
 
+	// Don't strip out debug information for eng builds.
+	if ctx.Config().Eng() {
+		r8Flags = append(r8Flags, "--debug")
+	}
+
 	return r8Flags, r8Deps
 }