[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.cpp b/adb/adb.cpp
index 06fdb69..aced079 100644
--- a/adb/adb.cpp
+++ b/adb/adb.cpp
@@ -1076,6 +1076,20 @@
     g_reject_kill_server = value;
 }
 
+static bool handle_mdns_request(std::string_view service, int reply_fd) {
+    if (!android::base::ConsumePrefix(&service, "mdns:")) {
+        return false;
+    }
+
+    if (service == "check") {
+        std::string check = mdns_check();
+        SendOkay(reply_fd, check);
+        return true;
+    }
+
+    return false;
+}
+
 HostRequestResult handle_host_request(std::string_view service, TransportType type,
                                       const char* serial, TransportId transport_id, int reply_fd,
                                       asocket* s) {
@@ -1320,6 +1334,10 @@
         return HostRequestResult::Handled;
     }
 
+    if (handle_mdns_request(service, reply_fd)) {
+        return HostRequestResult::Handled;
+    }
+
     return HostRequestResult::Unhandled;
 }