Fix FD leak in the Zygote.

Issues with LocalSocketImpl.close meant that a FD that was supposed to
be closed wasn't.  This CL switches to using Os.close to ensure that the
FD is closed properly.

Bug: 130309968
Bug: 130235718
Test: atest android.security.cts.FileDescriptorTest#testCLOEXEC
Test: Tested for file existance after Os.close call
Change-Id: I80fb3b378f651e71bc1ae9aab8636935a2ff6356
Merged-In: I80fb3b378f651e71bc1ae9aab8636935a2ff6356