[websession] Make WebsessionManager.Val.expiresAt available to plugins

The expiration time for a web session is stored in Val's private field
'expiresAt'. This value is needed for the websession-flatfile plugin in
order to implement the cleaning up of outdated web session cache files.

Create an accessor to WebsessionManager.Val.expiresAt field so that it
can be used by websession-flatfile plugin.

Change-Id: I2b98dbb7db0e12514adaa440b9774061ff368cb3

gerrit-httpd/src/main/java/com/google/gerrit/httpd/WebSessionManager.java

@@ -201,6 +201,10 @@ public class WebSessionManager {
       this.auth = auth;
     }
 
+    public long getExpiresAt() {
+      return expiresAt;
+    }
+
     Account.Id getAccountId() {
       return accountId;
     }