java libs that isn't installable is gracefully rejected

... rather than causing soong panic.

Bug: 146197571
Test: m (apex_test amended)
Change-Id: I3679a4e3041527533c1c05d54f3aa514abd6095b
diff --git a/apex/apex.go b/apex/apex.go
index be86d80..0aaba91 100644
--- a/apex/apex.go
+++ b/apex/apex.go
@@ -472,7 +472,7 @@
 }
 
 func (af *apexFile) Ok() bool {
-	return af.builtFile != nil || af.builtFile.String() == ""
+	return af.builtFile != nil && af.builtFile.String() != ""
 }
 
 type apexBundle struct {