Fix string-compare warning

packages/modules/DnsResolver/tests/resolv_private_dns_test.cpp:1096:51: error: result of comparison against a string literal is unspecified (use an explicit string comparison function instead) [-Werror,-Wstring-compare]
        EXPECT_EQ(doh.resumedConnections(), (flag == "1" ? 2 : 0));
                                                  ^  ~~~

Test: presubmit
Bug: 153764102
Change-Id: I82ba5615c75a9d0eb277ac3dc753e442169fc4d9