loader: cache top-of-chain PhysicalDevice

Bug: 27615575
If a layer wraps the VkPhysicalDevice object the
loader would be unable to figure out what VkInstance
it was associated with since it no longer matches
the VkPhysicalDevice queried from the driver.
Now cache the VkPhysicalDevice's from the top of
the chain as well to resolve this.

Change-Id: I7d04884dc3aa8623ce2f67e4f6575f74b82e2834