[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/adb_wifi.h b/adb/adb_wifi.h
index 585748c..49d3a91 100644
--- a/adb/adb_wifi.h
+++ b/adb/adb_wifi.h
@@ -27,6 +27,8 @@
                           std::string& response);
 bool adb_wifi_is_known_host(const std::string& host);
 
+std::string mdns_check();
+
 #else  // !ADB_HOST
 
 struct AdbdAuthContext;