feat(网络): 添加失去焦点时发送心跳包的功能

在NetworkManager和NetworkChannelBase中添加SetFocusHeartbeat方法，允许配置失去焦点时是否继续发送心跳包
移除NetworkComponent中不再使用的_isGaming变量

Author: AlianBlank

Runtime/Network/Network/NetworkManager.NetworkChannelBase.cs

@@ -972,6 +972,10 @@ public void SetIgnoreLogNetworkIds(List<int> sendIds, List<int> receiveIds)
                 IgnoreReceiveIds = receiveIds;
             }
 
+            /// <summary>
+            /// 设置是否开启心跳包失去焦点时也发送心跳包
+            /// </summary>
+            /// <param name="hasFocus">是否开启心跳包失去焦点时也发送心跳包</param>
             public void SetFocusHeartbeat(bool hasFocus)
             {
                 PFocusHeartbeat = hasFocus;

Runtime/Network/Network/NetworkManager.cs

@@ -246,6 +246,10 @@ public bool DestroyNetworkChannel(string channelName)
             return false;
         }
 
+        /// <summary>
+        /// 设置是否开启心跳包失去焦点时也发送心跳包
+        /// </summary>
+        /// <param name="hasFocus">是否开启心跳包失去焦点时也发送心跳包</param>
         public void SetFocusHeartbeat(bool hasFocus)
         {
             foreach (var networkChannel in m_NetworkChannels.Values)

Runtime/Network/NetworkComponent.cs

@@ -75,18 +75,13 @@ public int NetworkChannelCount
             get { return m_NetworkManager.NetworkChannelCount; }
         }
 
-        /// <summary>
-        /// 是否正在游戏中。
-        /// </summary>
-        private bool _isGaming = true;
 
         /// <summary>
         /// 应用程序获得或失去焦点时调用。
         /// </summary>
         /// <param name="hasFocus">应用程序是否获得焦点。</param>
         private void OnApplicationFocus(bool hasFocus)
         {
-            _isGaming = hasFocus;
             if (!m_FocusHeartbeat)
             {
                 return;