Revert "Revert c-ares as the default resolvre"

This reverts commit ca30b2240f.

src/core/ext/filters/client_channel/resolver/dns/c_ares/dns_resolver_ares.cc

@@ -478,7 +478,8 @@ static grpc_address_resolver_vtable ares_resolver = {
     grpc_resolve_address_ares, blocking_resolve_address_ares};
 
 static bool should_use_ares(const char* resolver_env) {
-  return resolver_env != nullptr && gpr_stricmp(resolver_env, "ares") == 0;
+  return resolver_env == nullptr || strlen(resolver_env) == 0 ||
+         gpr_stricmp(resolver_env, "ares") == 0;
 }
 
 void grpc_resolver_dns_ares_init() {

templates/test/cpp/naming/resolver_component_tests_defs.include

@@ -55,7 +55,6 @@ if cur_resolver and cur_resolver != 'ares':
       'needs to use GRPC_DNS_RESOLVER=ares.'))
   test_runner_log('Exit 1 without running tests.')
   sys.exit(1)
-os.environ.update({'GRPC_DNS_RESOLVER': 'ares'})
 os.environ.update({'GRPC_TRACE': 'cares_resolver'})
 
 def wait_until_dns_server_is_up(args,

test/core/client_channel/resolvers/dns_resolver_test.cc

@@ -75,7 +75,7 @@ int main(int argc, char** argv) {
   test_succeeds(dns, "dns:www.google.com");
   test_succeeds(dns, "dns:///www.google.com");
   char* resolver_env = gpr_getenv("GRPC_DNS_RESOLVER");
-  if (resolver_env == nullptr || gpr_stricmp(resolver_env, "native") == 0) {
+  if (resolver_env != nullptr && gpr_stricmp(resolver_env, "native") == 0) {
     test_fails(dns, "dns://8.8.8.8/8.8.8.8:8888");
   } else {
     test_succeeds(dns, "dns://8.8.8.8/8.8.8.8:8888");

test/cpp/naming/resolver_component_tests_runner.py

@@ -55,7 +55,6 @@ if cur_resolver and cur_resolver != 'ares':
       'needs to use GRPC_DNS_RESOLVER=ares.'))
   test_runner_log('Exit 1 without running tests.')
   sys.exit(1)
-os.environ.update({'GRPC_DNS_RESOLVER': 'ares'})
 os.environ.update({'GRPC_TRACE': 'cares_resolver'})
 
 def wait_until_dns_server_is_up(args,