Always call unbind.

Per a recent discussion unbind should always be called, even if
bind() returns false.

Restructured the NetworkScoreService code to allow for easier and
more robust testing of the inner ServiceConnection class.

Bug: 36999204
Test: runtest frameworks-services -c com.android.server.NetworkScoreServiceTest
Change-Id: I08870b7109cb7f51cdab8b632b7d1d0898c3c015