Remove FixturePreparer.Extend()

Use GroupFixturePreparers instead.

Bug: 182885307
Test: m nothing
Change-Id: Idc01d3cc5a57576a4cf417e9105d1ab851126e10
diff --git a/android/fixture_test.go b/android/fixture_test.go
index 8f04715..5b810e0 100644
--- a/android/fixture_test.go
+++ b/android/fixture_test.go
@@ -41,7 +41,7 @@
 
 	group := GroupFixturePreparers(preparer1, preparer2, preparer1, preparer1Then2)
 
-	extension := group.Extend(preparer4, preparer2)
+	extension := GroupFixturePreparers(group, preparer4, preparer2)
 
 	GroupFixturePreparers(extension, preparer1, preparer2, preparer2Then1, preparer3).Fixture(t)