statsd: fix double close.

Previously, fdopen was being called on a file descriptor that was owned
by a unique_fd without releasing. This leads to a double close, since
both fclose and the unique_fd destructor will try to close the fd.

Bug: http://b/113880863
Test: treehugger
Change-Id: I6f6f48d304861b5e4d7efee0d3ad0e30178a95a4