Make RTC coding flow support sub8x8 in key frame coding
Make RTC coding flow support sub8x8 in key frame coding

This commit enables the use of sub8x8 blocks in RTC key frame
encoding. It requires the block size to be preset and will decide
the coding mode and encode the bit-stream.

Change-Id: I35aaf8ee2d4d6085432410c7963f339f85a2c19b

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