libvulkan: ensure layer discovery is triggered only once for a new process

After decoupling layer discovery from driver loading, the layer discovery is
triggered at each call of vkCreateInstance, vkEnumerateInstanceLayerProperties
and vkEnumerateInstanceExtensionProperties. However, it takes non-trivial time
to traverse the layer search path for priviledged apps and non-updated system
apps. So this change just makes sure the layer discovery logic is triggered only
once for a new process.

Bug: 139443653
Bug: 135536511
Test: preload Vulkan and atest CtsGpuToolsHostTestCases
Change-Id: Ibe502fd4b089acbbff6f4a2485fa61c736a484b5