NsdManager: unit test coverage for servive registration

This patch adds test coverage for NsdManager#registerService() and
NsdManager#unregisterService(). This test shows a potential defect in
the api: if unregisterService() fails, the associated listener is always
unregistered from NsdManager. If the service initially registered is
still registered, this potentially make it impossible to unregister.

Test: added new unit test
Bug: 37013369, 33298084
Change-Id: Ia089b6d2f2a349907a8b29d9a3acd7f59e177887