[Vulkan] Expose instance extensions from implicit layers.

Previously the vulkan loader only exposes instance extensions from the
driver implementation. Per vkEnumerateInstanceExtensionProperties spec
the loader must also advertise instance extensions from implicitly
enabled layers.

Bug: b/143293104
Test: vkEnumerateInstanceExtensionProperties returns the instance
extensions from implicit layers.

Change-Id: I17f4ce370bf5f4bba295165a28836e2b7c03a318