Simplify socket configuration in DNSResponder::startServer()

Since Linux 2.6.27, sockets can be direclty created in non-blocking
mode.

Bug: 130686826
Test: atest resolv_integration_test
Merged-In: I574e3287df8915218ded60f010821eb9a75ae727
Merged-In: I04e890f2249f00cc74d2311711e291efa73a9a49
Change-Id: I82856f849154efafe1b88c4c94f4475c536afaab
(cherry picked from commit 1c54a438d35ebd4cb6eda530f717d53c4fef0104)