Fix crash in Vulkan WebView drawable when parent View has alpha

HWUI uses SkPaintFilterCanvas to apply View alpha in nested draw
calls. This conflicts with new SkDrawable::snapGpuDrawHandler
API used by Vulkan WebView. The new API works only with GPU
device (see SkGpuDevice::drawDrawable), which SkPaintFilterCanvas
does not have (it uses SkNoPixelsDevice instead).
This CL uses new Skia API to find wrapped GPU canvas and
draw SkDrawable on it.
Improve SKP capture for Vulkan WebView to match GL.

Test: Ran Fandango app with Vulkan. Captured a SKP.
Bug: 128792554
Change-Id: I27040347dc25c799b4e75f50526f426e9e33b663