Fix DnsTlsSocket fast shutdown path

Previously, DnsTlsSocket's destructor told the loop thread to
perform a clean shutdown by closing an IPC file descriptor.
However, the IPC file descriptor is now an eventfd, which does
not alert the listening thread when it is closed.

This change uses the eventfd counter's sign bit as an indication
that the destructor is requesting an immediate close.

Test: Includes regression test.
Bug: 123212403
Bug: 124058672
Change-Id: I6edc26bf504cbfbba7d055b1f8e52ac70e02c6e0