Make ADBD announce its presence over mDNS

We now request mdnsd from adb and register a service of type _adb._tcp
for clients to connect to.

Test: Verified service appears in avahi-browse
Bug: 28074466
(cherry picked from 379ac414e4d9f53388d903913022a55695292775)

Change-Id: Ie871b9c8b40e86063cc1e68e8f3e4290ead2d279