Change Entropy based Histogram Combine heuristic.

Don't combine the Histograms that have trivial (single valued A, R & B)
  symbols.
Following is the compression savings data along with compression time (before
& after) per image.
                     Before             After
                     bpp, rate(MP/s)    bpp, rate(MP/s)
Q=25, method = 4     2.508, 1.807       2.499, 1.916
Q=50, method = 4     2.460, 1.488       2.456, 1.512
Q=75, method = 4     2.452, 1.078       2.450, 1.092
Q=25, method = 5     2.505, 1.398       2.496, 1.383
Q=50, method = 5     2.458, 1.170       2.453, 1.143
Q=75, method = 5     2.453, 0.886       2.450, 0.855

This change provides 0.1-0.4% compression gains and speeds up the lossless
compression for the default method=4 (the drop in compression speed is between 1-3.5% for method=5).

Change-Id: Idfd88c2092f37afacd26a97097b3053f8183953a
Merge "Change Entropy based Histogram Combine heuristic."