Merge changes from topics "GL_screen_decor", "VK_screen_decor"

* changes:
  Plumb support for rendering A8 in Vulkan
  Plumb through A8 for GL/EGL
  Add COLOR_MODE_A8/ColorMode::A8
  Treat AHARDWAREBUFFER_FORMAT_R8_UNORM as kAlpha_8_SkAlphaType