adb: change mdns tls service names (RFC 6763).

Even though mdnsresponder seems to allow us to use _adb_secure_connect
and _adb_secure_pairing as service names, these names violate the syntax
outlined in RFC6763, and may not be compatible with other dns-sd
implementations.

Also address some comments from a previous CL.

Bug: 154268895

Test: atest adbd_test
Change-Id: Ia872e976fc4276587b500a827a41d46d9dc755dd
Merged-In: Ia872e976fc4276587b500a827a41d46d9dc755dd