use idle priority in visualbackendtest feature/skia

At least Skia and OpenGL use POST_PAINT priority for flushing drawn
contents to the screen. Using the higher REPAINT priority here could
mean that actually showing the contents would not happen. Also,
at least with Skia+Vulkan, it seems that Skia could queue commands
indefinitely, eventually running out of resources.

Change-Id: Ia3969bad18d710b006325a0fba11dc318ff93786