Remove libstatssocket from statsd_test

Statsd test links both libstatssocket and libstatssocket_private. It
shouldn't need to link both, but needs the private lib to be able to
access hidden APIs like getBuffer. So, remove it from statsd_test.

Test: atest statsd_test
Bug: 150435148
Change-Id: I46e71209489c8b1924800680f26e87ef7f4b8598