Ensure VmService instance is disposed after failed direct connection attempt (#66123)

_attemptServiceConnection in FallbackDiscovery would fail if the root
library URI was not a package URI even if the VM service connection
attempt was successful. This resulted in a VM service connection being
left alive, causing DDS to fail its connection to the VM service.

Updated _attemptServiceConnection to ensure the VM service instance is
disposed after a non-connection related failure and to allow for root
library URIs with a file scheme.