Use COEFF_CONTEXT_MASK to replace hard coded number
Use COEFF_CONTEXT_MASK to replace hard coded number

Change-Id: Ib192e5e5feab3a87112389d15d88d1a0102b1f1a

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