[driver_manager] Turn fragment driver URLs into constants

Also add `Device::is_fragment_device` and
`Device::is_fragment_proxy_device` convenience functions.

Change-Id: I6c34d15400016926d2caf5d2991027ca76053b09
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/773662
Fuchsia-Auto-Submit: Ian Fisher <iafisher@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: David Gilhooley <dgilhooley@google.com>