[normative]Set img's monochrome value correctly
[normative]Set img's monochrome value correctly

BUG=aomedia:2051

Change-Id: I666440d7a273106572ff2afdb895b9255cf363b9

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom