[CFL] Compute alpha costs using av1_cost_symbol
[CFL] Compute alpha costs using av1_cost_symbol

Use the uniform way to compue the cost of symbols in AV1.

Results on Subset1 (compared to 8a516a8f with CfL enabled)

   PSNR | PSNR Cb | PSNR Cr | PSNR HVS |    SSIM | MS SSIM | CIEDE 2000
-0.0357 | -0.0854 |  0.0305 |  -0.0422 | -0.0097 | -0.0171 |    -0.1042

Change-Id: Ie908fc7d20c480634002c78027b070223b3ea96d

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