How to stop head tracking (Google VR SDK 1.6)

Dear all,

We only have one camera and we want to stop head tracking when some conditions.
Our development environment is base on GVR SDK1.6 + Unity 5.6 1f1.

For stop head tracking, we set the camera rotation as zero when update() or lateupdate().
However, it's only working in the Unity game play mode.
The camera still start rotation when I build project as android application and run on a phone.
Has any way to stop head tracking on GVR SDK 1.6? or we missing something?

Thanks.