Move registration into android package

Mutator registration is tightly coupled with the android package, move
all registration from the soong package to the android package.

Test: build.ninja identical
Change-Id: Ie183d0b52cc7431c9e05b231934d189208ef1efe
diff --git a/android/module.go b/android/module.go
index 00219ae..d6eee44 100644
--- a/android/module.go
+++ b/android/module.go
@@ -19,7 +19,6 @@
 	"path/filepath"
 	"strings"
 
-	"android/soong"
 	"android/soong/glob"
 
 	"github.com/google/blueprint"
@@ -710,7 +709,7 @@
 }
 
 func init() {
-	soong.RegisterSingletonType("buildtarget", BuildTargetSingleton)
+	RegisterSingletonType("buildtarget", BuildTargetSingleton)
 }
 
 func BuildTargetSingleton() blueprint.Singleton {