Convert arch and os mutators to TransitionMutators

Replace archMutator and osMutator with TransitionMutators.

Bug: 319288033
Flag: EXEMPT refactor
Test: all soong tests pass
Test: no change to out/soong/*.ninja
Change-Id: I92a4d7c895dd79f1dd8064d0ca90e7010b563525
diff --git a/android/mutator.go b/android/mutator.go
index 2ef4d7f..18a9777 100644
--- a/android/mutator.go
+++ b/android/mutator.go
@@ -148,9 +148,9 @@
 }
 
 func registerArchMutator(ctx RegisterMutatorsContext) {
-	ctx.BottomUp("os", osMutator).Parallel()
+	ctx.Transition("os", &osTransitionMutator{})
 	ctx.Transition("image", &imageTransitionMutator{})
-	ctx.BottomUp("arch", archMutator).Parallel()
+	ctx.Transition("arch", &archTransitionMutator{})
 }
 
 var preDeps = []RegisterMutatorFunc{