Fix unsafety in SkiaShader storage, and texture unit accounting

bug:22390304

Fixes two issues:

1) The max texture unit wasn't large enough to handle the most complex
ComposeShader case (1 for draw primitive, 2 for gradient shader, 1 for
bitmap shader).

2) If a shader isn't supported by SkiaShader::store, the shader data
needs to be explicitly disabled, so we won't read uninitilized data
from it when trying to read shader information out.

Change-Id: I29ee7b7c1e07f67db88c1707bdc857daa305e713