Control mutator order

Register mutators inside lambdas that are called in a defined order to
correctly order mutators before and after the arch and deps mutators.

Test: build.ninja identical
Change-Id: Iefe2a3515aee8570e76a6e76925db4cda0e9e822

android/variable.go

@@ -24,7 +24,9 @@ import (
 )
 
 func init() {
-	RegisterBottomUpMutator("variable", variableMutator).Parallel()
+	PreDepsMutators(func(ctx RegisterMutatorsContext) {
+		ctx.BottomUp("variable", variableMutator).Parallel()
+	})
 }
 
 type variableProperties struct {