dht_proxy_server: fix Android push notifications

This commit updates the format of the push notification requests sent by
the DHT proxy server to conform to FCM's HTTP v1 API. (The previously
used API was deprecated in 2023, see:
https://firebase.google.com/docs/cloud-messaging/migrate-v1)
The "priority" and "ttl" (time to live) options are specific to Android
and therefore need to be put in the "android" block, as documented here:
https://firebase.google.com/docs/reference/fcm/rest/v1/projects.messages

Author: François-Simon Fauteux-Chapleau

src/dht_proxy_server.cpp

@@ -1107,10 +1107,14 @@ DhtProxyServer::sendPushNotification(const std::string& token, Json::Value&& jso
             notification["tokens"] = std::move(tokens);
             notification["platform"] = type == PushType::Android ? 2 : 1;
             notification["data"] = std::move(json);
-            notification["priority"] = highPriority ? "high" : "normal";
-            if (type == PushType::Android)
-                notification["time_to_live"] = 3600 * 24; // 24 hours
-            else {
+            auto priority = highPriority ? "high" : "normal";
+            if (type == PushType::Android) {
+                Json::Value androidConfig(Json::objectValue);
+                androidConfig["priority"] = priority;
+                androidConfig["ttl"] = "86400s"; // time to live = 24 hours
+                notification["android"] = std::move(androidConfig);
+            } else {
+                notification["priority"] = priority;
                 const auto expiration = std::chrono::system_clock::now() + std::chrono::hours(24);
                 uint32_t exp = std::chrono::duration_cast<std::chrono::seconds>(expiration.time_since_epoch()).count();
                 notification["expiration"] = exp;