Add Checks for mDNS Unregistration and Discovery Stop Operations

mDNS service unregistration and discovery stop should only occur
if the mDNS service has registered or started discovery.
Therefore, a check is added to ignore unregistration or stop
requests if registration or discovery has not been executed.

Test: atest CtsConnectivityMultiDevicesTestCases
Change-Id: If7becd5a89a45da977141e8f9767a8ed85d9ba1b
1 file changed