When a block only contains 2 unique colors, there's no need to run
K-means, as 'lb' and 'ub' can directly be used as the centroids.

Two benefits:
1. Encoding will be faster for this special case, and
2. Can potentially avoid sub-optimal/duplicate centroids generation
(due to integer rounding) when 'ub' and 'lb' values are very close.

Change-Id: Ice04fc8577e3cc8c74f87d623a16ac8de3257cdd