Currently, VRDeviceManager is typically created by lazily when VRServiceImpls connect to it, and it destroys itself when all VRServiceImpls go away.

However, it can be created in other ways, such as vrshelldelegate, and potentially by AR code that has yet to land.

We should have a clear contract on who can create it, and when it is destroyed, not simply destroyed implicitly when VRServiceImpls are gone.

An alternative is we could pass around VRDeviceManagerRef objects that will keep it alive, and it can self-destruct when all the VRDeviceManagerRefs are gone.  The key difference being other consumers such as VRShellDelegate don't need to be a VRServiceImpl to hold a ref or clean it up.