[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/adb.cpp b/adb/adb.cpp
index aced079..dcec0ba 100644
--- a/adb/adb.cpp
+++ b/adb/adb.cpp
@@ -1086,6 +1086,11 @@
         SendOkay(reply_fd, check);
         return true;
     }
+    if (service == "services") {
+        std::string services_list = mdns_list_discovered_services();
+        SendOkay(reply_fd, services_list);
+        return true;
+    }
 
     return false;
 }