Handle ART_USE_READ_BARRIER as a product variable.

Bug: 33492005
Test: top-level angler build.
Change-Id: I4e8a0a0af07fec0a5764692d8340faf4c9bcf2b4
diff --git a/build/art.go b/build/art.go
index 0af1767..e6e0544 100644
--- a/build/art.go
+++ b/build/art.go
@@ -58,7 +58,7 @@
 		asflags = append(asflags, "-DART_HEAP_POISONING=1")
 	}
 
-	if envTrue(ctx, "ART_USE_READ_BARRIER") {
+	if envTrue(ctx, "ART_USE_READ_BARRIER") || ctx.AConfig().ArtUseReadBarrier() {
 		// Used to change the read barrier type. Valid values are BAKER, BROOKS, TABLELOOKUP.
 		// The default is BAKER.
 		barrierType := envDefault(ctx, "ART_READ_BARRIER_TYPE", "BAKER")