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.bp b/Android.bp
index 0202ea7..0ba669c 100644
--- a/Android.bp
+++ b/Android.bp
@@ -45,7 +45,6 @@
     ],
     srcs: [
         "doc.go",
-        "register.go",
     ],
 }
 
@@ -73,6 +72,7 @@
         "android/onceper.go",
         "android/package_ctx.go",
         "android/paths.go",
+        "android/register.go",
         "android/util.go",
         "android/variable.go",