Restrict libhidltransport entirely.

Now that users have been cleaned up (in Android.bp). When remaining
users in Android.mk are cleaned up and prebuilts are cleaned up, the
library can be removed.

Bug: 135686713
Test: compile (checks visibility)
Change-Id: I912b71bbe3d1df371edef1b7369ea3596d79238f
diff --git a/Android.bp b/Android.bp
index 27988cb..3ab4310 100644
--- a/Android.bp
+++ b/Android.bp
@@ -112,7 +112,5 @@
     name: "libhidltransport",
     vendor_available: true,
 
-    visibility: [
-        "//vendor:__subpackages__",
-    ],
+    visibility: [":__subpackages__"],
 }