[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_wifi.h b/adb/adb_wifi.h
index 49d3a91..3a6b0b1 100644
--- a/adb/adb_wifi.h
+++ b/adb/adb_wifi.h
@@ -28,6 +28,7 @@
 bool adb_wifi_is_known_host(const std::string& host);
 
 std::string mdns_check();
+std::string mdns_list_discovered_services();
 
 #else  // !ADB_HOST