Stop using NSD_ON setting

NSD_ON setting has never been set since it created. Besides,
CTS tests assume that nsd service is always enabled otherwise
the tests will fail. Thus, remove the setting from NsdService
which is no need to read/write it and it's also a hidden symbols.

Bug: 206702844
Test: atest FrameworksNetTests CtsNetTestCases
Change-Id: Id3cef7def96925cec5095e12910e9b97ba7efb11