Add unconverted reason for jarjar incompatibility.

Test: m bp2build
Change-Id: I112b84fa53f57b9df2bd1e1616012fe55b48001e
diff --git a/java/aar.go b/java/aar.go
index 180e1d7..20972b2 100644
--- a/java/aar.go
+++ b/java/aar.go
@@ -1305,7 +1305,11 @@
 }
 
 func (a *AndroidLibrary) ConvertWithBp2build(ctx android.TopDownMutatorContext) {
-	commonAttrs, bp2buildInfo := a.convertLibraryAttrsBp2Build(ctx)
+	commonAttrs, bp2buildInfo, supported := a.convertLibraryAttrsBp2Build(ctx)
+	if !supported {
+		return
+	}
+
 	depLabels := bp2buildInfo.DepLabels
 
 	deps := depLabels.Deps