Move f/b/packages/Nsd files to f/b/packages/ConnectivityT

Move Nsd files to ConnectivityT so that nsd, ipsec,
ethernet and netstats files can live in the same
directory and it will be easier to move everything
in frameworks/base/packages/ConnectivityT to
packages/modules/Connectivity after fixing all of the
hidden API usages.

Bug: 204153604
Test: FrameworksNetTests
      CtsNetTestCases
Change-Id: I411e242e8739d15920cfc2fe274115f7f39bd89f