Fixed issue 878 where `XROcclusionSubsystem` would use `Provider.environmentDepthCpuImageApi` instead of `Provider.environmentDepthConfidenceCpuImageApi` when acquiring the Environment Depth Confidence CPU Image. This did not affect the provider implementations of Google ARCore and Apple ARKit packages because these providers use singleton `XRCpuImage.Api` for all CPU image types. However, it could be an issue for a custom AR provider that uses different instances of `XRCpuImage.Api` for different CPU image types.
