Add VDM#getDisplayNameForPersistentDeviceId API

Automatically setting display name for CDM associations
created via the shell command because we require them to
be non-null in the tests. Also, this is closer to reality
because all self-managed associations require display names.

Fix: 315481938
Test: atest VirtualDeviceManagerServiceTest

Change-Id: Icb6e23a86910a68f768f244dda3ae4a3e2fa0f59