align scalability_structure code with spec
align scalability_structure code with spec

Modifications were made to spec, but not reflected in SW

BUG=aomedia:1803

Change-Id: I1dbd518ab1538dbff3c4b6b96c5dd29da1e7b6b3

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