[adb client] Add "adb mdns check" command.

This command will check if the mdns daemon is available on the host
machine.

Bug: 152510294

Test: pkill -9 mdnsd; adb mdns check; mdnsd; adb mdns check;
Test: test_adb.py
Change-Id: If644678a339763817a8a7adcbdc545626d161aba
diff --git a/adb/client/commandline.cpp b/adb/client/commandline.cpp
index 29f9dc1..7b8b2d7 100644
--- a/adb/client/commandline.cpp
+++ b/adb/client/commandline.cpp
@@ -127,6 +127,7 @@
         "       localfilesystem:<unix domain socket name>\n"
         " reverse --remove REMOTE  remove specific reverse socket connection\n"
         " reverse --remove-all     remove all reverse socket connections from device\n"
+        " mdns check               check if mdns discovery is available\n"
         "\n"
         "file transfer:\n"
         " push [--sync] [-z ALGORITHM] [-Z] LOCAL... REMOTE\n"
@@ -1910,6 +1911,25 @@
 
         ReadOrderlyShutdown(fd);
         return 0;
+    } else if (!strcmp(argv[0], "mdns")) {
+        --argc;
+        if (argc < 1) error_exit("mdns requires an argument");
+        ++argv;
+
+        std::string error;
+        if (!adb_check_server_version(&error)) {
+            error_exit("failed to check server version: %s", error.c_str());
+        }
+
+        std::string query = "host:mdns:";
+        if (!strcmp(argv[0], "check")) {
+            if (argc != 1) error_exit("mdns %s doesn't take any arguments", argv[0]);
+            query += "check";
+        } else {
+            error_exit("unknown mdns command [%s]", argv[0]);
+        }
+
+        return adb_query_command(query);
     }
     /* do_sync_*() commands */
     else if (!strcmp(argv[0], "ls")) {