libhidltransport: restrict visibility

It's empty, and in order to reclaim 4kb usage per product, we need to
start restricting this.

Bug: 135686713
Test: build (checks visibility)
Change-Id: Idc979ddc124b5a3b775a2e992eaca6038047d7fc
diff --git a/Android.bp b/Android.bp
index 819732d..4ff1558 100644
--- a/Android.bp
+++ b/Android.bp
@@ -111,4 +111,9 @@
 cc_library {
     name: "libhidltransport",
     vendor_available: true,
+
+    visibility: [
+        "//device:__subpackages__",
+        "//vendor:__subpackages__",
+    ],
 }