Keep the native mdns daemon alive for pre-S application

Roll back the behavior changes by checking the target SDK to ensure that
there are no compatibility issues with the pre-S application.
If the target SDK of the application <S, NsdManager will actively send a
cmd to start the native daemon, and NsdService will keep the daemon
until the last client with the target SDK <S disconnects.

Test: atest NsdManagerTest NsdServiceTest
Bug: 191844585
Change-Id: Ie93d5d585e126fe220ae865bbc7274f21a925984