This reduces the priority to best effort (from the default that seems to be best effort), and allows us to run SSIM even on 8k24fps video. Without this CL,
start()'ing a second codec may result in a MediaCodec.CodecException.
Tested to confirm that transformation8k24():
* fails deterministically without this CL, or with KEY_PRIORITY set to 0.
* succeeds deterministically after this CL (~18s on Samsung Z Fold 4)
PiperOrigin-RevId: 490570416
