Fix prebuilt mutator ordering in tests

Previously, the prebuilt mutators were added by the
cc.RegisterRequiredBuildComponentsForTest() function as a convenience
but unfortunately it lead to some of the mutators being in a different
order in the tests than in the normal build.

This change:
* Extracts the RegisterPrebuiltMutators() call from
  cc.RegisterRequiredBuildComponentsForTest()
* Makes sure that the prebuilt mutators are registered before the
  visibility gatherer and enforcer mutators.

Bug: 162505935
Test: m nothing
Change-Id: I7d959b558200b502f0a5e4653c41ea01414e142a
diff --git a/sysprop/sysprop_test.go b/sysprop/sysprop_test.go
index 8503386..711129c 100644
--- a/sysprop/sysprop_test.go
+++ b/sysprop/sysprop_test.go
@@ -67,6 +67,8 @@
 		ctx.BottomUp("sysprop_deps", syspropDepsMutator).Parallel()
 	})
 
+	android.RegisterPrebuiltMutators(ctx)
+
 	cc.RegisterRequiredBuildComponentsForTest(ctx)
 	ctx.PreDepsMutators(func(ctx android.RegisterMutatorsContext) {
 		ctx.BottomUp("sysprop_java", java.SyspropMutator).Parallel()