Do listening on dnsproxyd using only one thread.

This helps to prevent a race condition where android_getaddrinfo_proxy
and android_gethostbyaddr_proxy both try to write to the socket at the
same time.

Change-Id: If3c286011a31aec972b4c47af6b616ab1e65b560
Signed-off-by: Evan McClain <aeroevan@gmail.com>