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/variable.go b/android/variable.go
index d2dc5b9..531125d 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -24,7 +24,7 @@
 )
 
 func init() {
-	RegisterBottomUpMutator("variable", variableMutator)
+	RegisterBottomUpMutator("variable", variableMutator).Parallel()
 }
 
 type variableProperties struct {