Mark update engine binder interfaces as hidden

After generated .java files are prevented from escaping their directories,
this trips up the system api verification.

Bug: 29619260
Change-Id: If6c47fd118056b5f7f8b74b17e81d7556cd60bac
diff --git a/binder_bindings/android/os/IUpdateEngineCallback.aidl b/binder_bindings/android/os/IUpdateEngineCallback.aidl
index 729abdd..ee15c8b 100644
--- a/binder_bindings/android/os/IUpdateEngineCallback.aidl
+++ b/binder_bindings/android/os/IUpdateEngineCallback.aidl
@@ -16,7 +16,10 @@
 
 package android.os;
 
+/** @hide */
 oneway interface IUpdateEngineCallback {
+  /** @hide */
   void onStatusUpdate(int status_code, float percentage);
+  /** @hide */
   void onPayloadApplicationComplete(int error_code);
 }