Change Vulkan swapchain image count to 3

Using only 2 swapchain images causes performance issues on Android

ProjectSettings/ProjectSettings.asset

   switchNVNShaderPoolsGranularity: 33554432
   switchNVNDefaultPoolsGranularity: 16777216
   switchNVNOtherPoolsGranularity: 16777216
-  vulkanNumSwapchainBuffers: 2
+  vulkanNumSwapchainBuffers: 3
   vulkanEnableSetSRGBWrite: 0
   m_SupportedAspectRatios:
     4:3: 1