Remove FixturePreparer.Extend()

Use GroupFixturePreparers instead.

Bug: 182885307
Test: m nothing
Change-Id: Idc01d3cc5a57576a4cf417e9105d1ab851126e10
diff --git a/genrule/genrule_test.go b/genrule/genrule_test.go
index 2ee456d..3f1e9f3 100644
--- a/genrule/genrule_test.go
+++ b/genrule/genrule_test.go
@@ -670,7 +670,8 @@
 			cmd: "cat $(in) > $(out)",
 		}
        `
-	result := prepareForGenRuleTest.Extend(
+	result := android.GroupFixturePreparers(
+		prepareForGenRuleTest,
 		android.FixtureModifyConfigAndContext(
 			func(config android.Config, ctx *android.TestContext) {
 				config.TestProductVariables.Allow_missing_dependencies = proptools.BoolPtr(true)