Fix SocketListener socket leak issue.

The problem was: if a socket is shared between SocketListener and another
thread, only if the last reference is removed by SocketListener can the socket
be closed, otherwise the socket will leak. This sometimes happens in netd's
dnsproxyd.

This change let the SocketClient own the socket and close the socket when
the SocketClient is destructed.

Change-Id: I2865fbfe9ee4d8b3e43d7e02919dbb2d261f70de