I tried setting the orientation to Sensor instead of Landscape to fix another problem but it crashes the app.
Steps:
1-Make a new project. I used the FPS template.
2-Disable all VR plugins except Google VR.
3-Configure your project’s Android settings like this:
Min SDK and Target SDK: 21
Orientation Sensor
4-Set build configuration to Shipping
5-Android SDK to ndk-r11c with SDK API level and NDK API level to android-21
Build the project and send it to the device.
Hold the device in portrait mode while it starts up.
When the Google Cardboard prompt appears, hold the device in landscape mode.
Switch to portrait mode right after that and the app crashes.
I was able to replicate the bug in an empty project I uploaded here: