Fix SIGSEGV crash on Android startup: guard against null surface handle

Three-layer defense against null pointer dereference (SIGSEGV at pc=0x0)
when the Android native surface is not yet ready during Vulkan initialization:

1. VeldridDevice.cs: Poll-wait up to 5s for Android surface handle to become
   non-zero before creating Vulkan swapchain
2. VkSurfaceUtil.cs: Guard against null ANativeWindow from ANativeWindow_fromSurface
3. DrawThread.cs: Wrap initial BeginFrame/FinishFrame in try-catch

Co-authored-by: winnerspiros <1675249+winnerspiros@users.noreply.github.com>

Author: Copilot