fix WBMP image will get null color table when draw in RGB565 Canvas

fix WBMP image will get null color table when draw in RGB565 Canvas

Symptom: draw Index8 to RGB565 will using SkColorTable::lock16BitCache and return a null array.
Root Cause:lock16BitCache return null pointer because default type is kPremul_SkAlphaType
Solution: WBMP color table should be kOpaque_SkAlphaType
Reproduce steps: draw a WBMP with a matrix that apply rotate and transform

This patch was proposed by arthur_hung at htc.com to the AOSP project.

R=reed@google.com, scroggo@google.com

Review URL: https://codereview.chromium.org/377443007