Move the libnetworkstacktestsjni target to its own directory.

The point of this is that now atest understands that the classes
in NetworkStackTests are only in NetworkStackTests, so it stops
asking for the right module every time.

You may have to call atest with --rebuild-module-info once.

Test: atest now does not ask for the module when running,
      say, NetTestUtilsTests

Change-Id: I63a38b34667be55cbcd0a80f17225ac109cdc919