Add vr_hwc to the system-background cpuset also.

We've added hwcomposer to the system-background cpuset, and vr_hwc
should be there too.

Bug: 75343056
Test: Took a 10s systrace of a YouTube VR video and saw no dropped
frames.

Change-Id: I2a623af839e75298b26fc6b93da0ecbf89b9b666