[adb client] Add "mdns services" command.

This command list all discovered mdns services, so we
can connect via service name later on.

Bug: 152521166

Test: 'adb mdns services'
Test: test_adb.py
Change-Id: I23d42a7933e67a65bd0c9924afd6abe5915c0a11
diff --git a/adb/client/commandline.cpp b/adb/client/commandline.cpp
index 7b8b2d7..d565e01 100644
--- a/adb/client/commandline.cpp
+++ b/adb/client/commandline.cpp
@@ -128,6 +128,7 @@
         " 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"
+        " mdns services            list all discovered services\n"
         "\n"
         "file transfer:\n"
         " push [--sync] [-z ALGORITHM] [-Z] LOCAL... REMOTE\n"
@@ -1925,6 +1926,10 @@
         if (!strcmp(argv[0], "check")) {
             if (argc != 1) error_exit("mdns %s doesn't take any arguments", argv[0]);
             query += "check";
+        } else if (!strcmp(argv[0], "services")) {
+            if (argc != 1) error_exit("mdns %s doesn't take any arguments", argv[0]);
+            query += "services";
+            printf("List of discovered mdns services\n");
         } else {
             error_exit("unknown mdns command [%s]", argv[0]);
         }