NsdService: disentangle NativeDaemonConnector from ctor

This patch disentangles the creation of NativeDaemonConnector from the
creation of NsdService by introducing a wrapper type DaemonConnection.

This allows to mock this dependency in unit tests.

Test: enhanced previously introduced new tests.
Bug: 37013369, 33298084
Change-Id: I894b7cddfc509f86d0134d214c88c063b81b5e0a