Use empty string for core image variant

Use the empty string for the core image variant so that modules
added to imageMutator do not change their build directory.

Bug: 142286466
Test: m checkbuild
Change-Id: Ida4534d9a4d6176236aaa480fed359ce27acfaa1
Merged-In: Ida4534d9a4d6176236aaa480fed359ce27acfaa1
(cherry picked from commit 72d685ee7f45e5393be44ae4159edf083ac918de)
diff --git a/xml/xml_test.go b/xml/xml_test.go
index f42ba77..0a11566 100644
--- a/xml/xml_test.go
+++ b/xml/xml_test.go
@@ -104,7 +104,7 @@
 		{rule: "xmllint-minimal", input: "baz.xml"},
 	} {
 		t.Run(tc.schemaType, func(t *testing.T) {
-			rule := ctx.ModuleForTests(tc.input, "android_arm64_armv8-a_core").Rule(tc.rule)
+			rule := ctx.ModuleForTests(tc.input, "android_arm64_armv8-a").Rule(tc.rule)
 			assertEqual(t, "input", tc.input, rule.Input.String())
 			if tc.schemaType != "" {
 				assertEqual(t, "schema", tc.schema, rule.Args[tc.schemaType])
@@ -112,6 +112,6 @@
 		})
 	}
 
-	m := ctx.ModuleForTests("foo.xml", "android_arm64_armv8-a_core").Module().(*prebuiltEtcXml)
+	m := ctx.ModuleForTests("foo.xml", "android_arm64_armv8-a").Module().(*prebuiltEtcXml)
 	assertEqual(t, "installDir", buildDir+"/target/product/test_device/system/etc", m.InstallDirPath().String())
 }