Add fixes & test for isConfigTexturable and isConfigRenderable

Add fixes & test for isConfigTexturable and isConfigRenderable

This CL fixes:
  isConfigTexturable was returning true for:
            kRG_float for ANGLE ES2 configs
  isConfigRenderable was returning true for:
            kAlpha_8 for ANGLE ES2 configs
  isConfigTexturable and isConfigRenderable were returning true for:
            SBGRA on ES2
  The NexusPlayer was marking RGBA & RG float configs as renderable but not textureable

Bug: 720325

Change-Id: If21361870dbdde8f3e09bc9dff3a394f2a329157
Reviewed-on: https://skia-review.googlesource.com/17387
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>