Make apexMutator parallel

apexMutator only modifies the currently visited module, it can
visit modules in parallel.

Test: builds
Change-Id: I7d0ad142f5161742bd25dc57bac64c0f4a733ecf

apex/apex.go

@@ -153,7 +153,7 @@ func init() {
 
 	android.PostDepsMutators(func(ctx android.RegisterMutatorsContext) {
 		ctx.TopDown("apex_deps", apexDepsMutator)
-		ctx.BottomUp("apex", apexMutator)
+		ctx.BottomUp("apex", apexMutator).Parallel()
 	})
 }