[Vulkan] Expose device extensions from implicit layers.

Previously the vulkan loader only exposes device extensions from the
driver implementation when the layer name is not specified. Per
vkEnumerateDeviceExtensionProperties spec the loader must also advertise
device extensions from implicitly enabled layers.

Bug: b/143293104
Test: atest android.gputools.cts.CtsRootlessGpuDebugHostTest
Change-Id: Iaf5786ba7e371a290ecd1764af69b5298371cfdd