[mdns] allow hostnames beginning with a digit

A Matter device uses a hex as the hostname so it may begin with a digit
which wasn't allowed in NsdService. This CL looses this check to allow
Matter devices to register their services at Android Thread Border
Router.

Bug: 334828525
Test: CI
Change-Id: Ia779791e214158a08c2ec06adf8d5630ec8ea616
2 files changed