Facilitate decoder row multi-threading
Facilitate decoder row multi-threading

Abstract the functions to separate bitstream parsing and decoding
stage for row based multi-threading of decoding stage.

Change-Id: I4f5d0fda12d643378370b648cfde7928984ac6b6

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