Add flags for switching RE to Vulkan and Graphite

We are working towards shipping RenderEngine using Skia's Graphite
Vulkan backend, so add a flag for that. In addition, add a flag for
switching RenderEngine to use Vulkan while still using Skia's older
Ganesh backend. This will allow us to get more soak time with Vulkan
while we get Graphite working here.

Bug: 293371537
Test: NA
Change-Id: Id1e8db7a33b8dbffb9e713bd2d8afa4ca5dbf8a9