connection keepAlive does not apply.

### [REQUIRED] Step 2: Describe your environment

* Operating System version: linux
* Firebase SDK version: firebase-admin-8.1.0.jar
* Library version: 8.1.0.jar
* Firebase Product: Cloud Messaging (auth, database, storage, etc)

### [REQUIRED] Step 3: Describe the problem
We confirmed that keepAlive is applied when using firebase-admin version 6.8.0.
However, after upgrading to firebase-admin 8.1.0, keepAlive is not applied and the connection is immediately disconnected.

#### Steps to reproduce:
firebase-admin 6.8.0 FirebaseMessagingClient.sendSingleRequest()
(apache httpclient-4.5.11) ConnectionHolder.releaseConnection() is called due to line 129.
At this time, this.released changes from false to true.
After that, ApiClientUtils.disconnectQuietly(response) -> (apache httpclient-4.5.11) ConnectionHolder.abortConnection() on line 132 is called, but the connection is not disconnected because ConnectionHolder.released = true.

However, in firebase-admin 8.1.0, ConnectionHolder.releaseConnection() is not called, but ConnectionHolder.abortConnection() is called, so the connection is disconnected.

What happened? How can we make the problem occur?
Establishing connections is expensive. Therefore, recycling connections for a certain period of time can help improve performance. According to what I have checked, the code of the latest version, 9.2.0, is also the same.

#### Relevant Code:

```
private FirebaseMessaging initializeFirebaseMessaging()
throws IOException {
CloseableHttpClient httpClient = newHttpClient(proxyHost, proxyPort);

HttpTransport httpTransport = new ApacheHttpTransport(httpClient);

FirebaseOptions options = FirebaseOptions.builder()
// need credentials etc
.setHttpTransport(httpTransport)
.build();

FirebaseApp.initializeApp(options);
return FirebaseMessaging.getInstance();
}

private CloseableHttpClient newHttpClient(String proxyHost, int proxyPort) {
HttpClientBuilder httpClientBuilder = ApacheHttpTransport.newDefaultHttpClientBuilder()
.evictExpiredConnections()
.setKeepAliveStrategy(
new CustomConnectionKeepAliveStrategy(
TimeUnit.MINUTES.toMillis(3)))
.setConnectionTimeToLive(10, TimeUnit.MINUTES)
.setDefaultRequestConfig(requestConfig())
.disableCookieManagement();
return httpClientBuilder.build();
}

public class CustomConnectionKeepAliveStrategy implements ConnectionKeepAliveStrategy {

public static final CustomConnectionKeepAliveStrategy INSTANCE =
new CustomConnectionKeepAliveStrategy(TimeUnit.SECONDS.toMillis(50),
DefaultConnectionKeepAliveStrategy.INSTANCE);

public static final ConnectionKeepAliveStrategy NO_KEEPALIVE = (response, context) -> 0;

private static final long KEEP_ALIVE_SAFE_GAP = 500L;
private final ConnectionKeepAliveStrategy defaultConnectionKeepAliveStrategy;
private final long defaultKeepAliveTimeoutInMillis;

public CustomConnectionKeepAliveStrategy(long defaultKeepAliveTimeoutInMillis) {
this(defaultKeepAliveTimeoutInMillis, DefaultConnectionKeepAliveStrategy.INSTANCE);
}

public CustomConnectionKeepAliveStrategy(long defaultKeepAliveTimeoutInMillis,
ConnectionKeepAliveStrategy defaultConnectionKeepAliveStrategy) {
Args.notNegative(defaultKeepAliveTimeoutInMillis, "defaultKeepAliveTimeoutInMillis");
Args.notNull(defaultConnectionKeepAliveStrategy, "defaultConnectionKeepAliveStrategy");
this.defaultKeepAliveTimeoutInMillis = defaultKeepAliveTimeoutInMillis;
this.defaultConnectionKeepAliveStrategy = defaultConnectionKeepAliveStrategy;
}

public static long adjustKeepAliveTimeout(long keepAliveTimeoutInMillis) {
return Math.max(0L, (keepAliveTimeoutInMillis - KEEP_ALIVE_SAFE_GAP));
}

@Override
public long getKeepAliveDuration(final HttpResponse response, final HttpContext context) {
long keepAliveDuration = defaultConnectionKeepAliveStrategy.getKeepAliveDuration(response, context);
if (keepAliveDuration == -1L) {
return defaultKeepAliveTimeoutInMillis;
}
if (keepAliveDuration > 0L) {
return adjustKeepAliveTimeout(keepAliveDuration);
}
return keepAliveDuration;
}
```

まず、firebase-admin 6.8.0 と 8.1.0 における FirebaseMessagingClient.sendSingleRequest() の動作を比較し、ApiClientUtils.disconnectQuietly(response) と、レポートで説明されている Apache HttpClient ConnectionHolder の呼び出しに注目します。提供されたカスタム keep-alive 戦略を使用して接続のライフサイクルを再現し、設定された期間に従って接続が保持および再利用されることを確認します。

索引モデルが issue の本文から書いたものです。