Set sensor-supplied orientation always to portrait in VrMode.

For VrMode, we report the sensor as always being in
the device's default orientation so that:
1) The orientation doesn't change as the user moves their head.
2) 2D UI within VR shows in the device's default orientation.
This only overwrites the sensor-provided orientation and does not
affect any explicit orientation preferences specified by any activities.

Test: Start 2D UI within VR while device is oriented in landscape and
ensure that the item is displayed in portrait.  Also start
2D UI with custom orientation and ensure it starts in its
preferred orientation.

Bug: 36516916
Change-Id: I56380b860eec73cf9485185ded811626e40c5c57