Move bufferhubd into system_ext

it is not a part of the system according to
generic_system.mk.

Bug: 173538619
Bug: 167689596
Test: presubmit
Signed-off-by: Roman Kiryanov <rkir@google.com>
Change-Id: If7bbb5d5c19ccec8e510d0a4f1618737e503a794
diff --git a/services/vr/bufferhubd/Android.bp b/services/vr/bufferhubd/Android.bp
index cfaa368..7097e7a 100644
--- a/services/vr/bufferhubd/Android.bp
+++ b/services/vr/bufferhubd/Android.bp
@@ -47,6 +47,7 @@
 
 cc_binary {
     srcs: ["bufferhubd.cpp"],
+    system_ext_specific: true,
     cflags: [
         "-DLOG_TAG=\"bufferhubd\"",
         "-DTRACE=0",