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/cmd/soong_build/main.go b/cmd/soong_build/main.go
index 5b0c83c..ed8f2fd 100644
--- a/cmd/soong_build/main.go
+++ b/cmd/soong_build/main.go
@@ -22,8 +22,6 @@
 
 	"github.com/google/blueprint/bootstrap"
 
-	"android/soong"
-
 	"android/soong/android"
 )
 
@@ -33,7 +31,7 @@
 	// The top-level Blueprints file is passed as the first argument.
 	srcDir := filepath.Dir(flag.Arg(0))
 
-	ctx := soong.NewContext()
+	ctx := android.NewContext()
 
 	configuration, err := android.NewConfig(srcDir, bootstrap.BuildDir)
 	if err != nil {