Make BottomUpMutators parallel

Append .Parallel() to all of the RegisterBottomUpMutator calls to tell
Blueprint it can run them in parallel.

Test: identical build.ninja, passes race detector
Change-Id: I969a0689522d4cba7c8ff51e2aa00fe2fd338a89
diff --git a/android/mutator.go b/android/mutator.go
index 9405f2d..c0cb3aa 100644
--- a/android/mutator.go
+++ b/android/mutator.go
@@ -44,8 +44,8 @@
 	androidBaseContextImpl
 }
 
-func RegisterBottomUpMutator(name string, mutator AndroidBottomUpMutator) {
-	soong.RegisterBottomUpMutator(name, func(ctx blueprint.BottomUpMutatorContext) {
+func RegisterBottomUpMutator(name string, mutator AndroidBottomUpMutator) soong.BottomUpMutatorHandle {
+	return soong.RegisterBottomUpMutator(name, func(ctx blueprint.BottomUpMutatorContext) {
 		if a, ok := ctx.Module().(Module); ok {
 			actx := &androidBottomUpMutatorContext{
 				BottomUpMutatorContext: ctx,