Match aomdec Y4M output for colocated chroma

Use C420 instead of C420paldv.

Note: the Y4M2 format is poorly-specified and not all decoders may output the expected results. ffmpeg for example treats C420 as equivalent C420jpeg (block-centered samples). C422 and C444 have colocated samples in the original format documentation, but C420 is not formally defined, despite being the only suitable label for colocated chroma in 4:2:0. In aomdec, a warning is output to the user in order to warn of potential mismatched output. C420paldv is not used, because "it has alternating Cb and Cr components, whereas in the AV1 spec, colocated does not have them alternating".

Reference: https://bugs.chromium.org/p/aomedia/issues/detail?id=2096