Update variable name in error messages.

Variable PRODUCT_UPDATABLE_SYSTEM_SERVER_JARS was renamed to
PRODUCT_APEX_SYSTEM_SERVER_JARS in https://r.android.com/1779472.

Bug: 191127295
Test: m nothing
Change-Id: I0c8ae839cab443c824760b40d30b4374982010f9
diff --git a/apex/systemserver_classpath_fragment_test.go b/apex/systemserver_classpath_fragment_test.go
index dc682fa..412fa0e 100644
--- a/apex/systemserver_classpath_fragment_test.go
+++ b/apex/systemserver_classpath_fragment_test.go
@@ -138,7 +138,7 @@
 		dexpreopt.FixtureSetApexSystemServerJars("myapex:foo"),
 	).
 		ExtendWithErrorHandler(android.FixtureExpectsAtLeastOneErrorMatchingPattern(
-			`in contents must also be declared in PRODUCT_UPDATABLE_SYSTEM_SERVER_JARS`)).
+			`in contents must also be declared in PRODUCT_APEX_SYSTEM_SERVER_JARS`)).
 		RunTestWithBp(t, `
 			apex {
 				name: "myapex",