Use HWC_DISPLAY_PRIMARY and _EXTERNAL

Some of the code in the SurfaceFlinger and VR composer assume that the
composer has display IDs HWC_DISPLAY_PRIMARY and HWC_DISPLAY_EXTERNAL
for those specific displays. Modify test to follow the requirement.

Bug: 64446190
Test: Run the test on Marlin
Change-Id: I7fef17158d62bd7cd242b97d37724a229f9c7418