notif: Handle RemoveFcmMessage.

This implementation has some problems.

* Group Notification doesn't get cancelled when its children are.
  It will appear as an empty notification if all notification
  within a group are cancelled and will be required to be manually
  removed.
* This will only happen in case of multiple groups, I have managed
  to cancel all notification if there are only group notifications
  remaining as active notifications.
* It would be nice to have more information in notification
  payload to remove notification, the implementation currently
  performs a linear search for every message to be removed.
  Consider using a better approach.

android/app/src/main/java/com/zulipmobile/notifications/FCMPushNotifications.kt

@@ -13,6 +13,7 @@ import android.graphics.Color
 import android.net.Uri
 import android.os.Build
 import android.os.Bundle
+import android.service.notification.StatusBarNotification
 import android.util.Log
 import androidx.core.app.NotificationManagerCompat
 import androidx.core.app.NotificationCompat
@@ -71,7 +72,32 @@ internal fun onReceived(context: Context, mapData: Map<String, String>) {
 
     if (fcmMessage is MessageFcmMessage) {
         updateNotification(context, fcmMessage)
-    } // TODO handle case for RemoveFcmMessage
+    } else if (fcmMessage is RemoveFcmMessage) {
+        removeNotification(context, fcmMessage)
+    }
+}
+
+private fun removeNotification(context: Context, fcmMessage: RemoveFcmMessage) {
+    for (fcmMessageId in fcmMessage.messageIds) {
+        val activeNotifications = getActiveNotification(context) ?: return
+        for (statusBarNotification in activeNotifications) {
+            val notification = statusBarNotification.notification
+            val messageId = notification.extras.getInt("zulipMessageId")
+            if (fcmMessageId == messageId) {
+                NotificationManagerCompat.from(context).cancel(statusBarNotification.tag, statusBarNotification.id)
+                break
+            }
+        }
+    }
+    // TODO figure out a way to cancel individual group notifications
+    // misbehaves with multiple realm, shows empty notification.
+    // This cancels all group Notification when no other notification
+    // is present.
+    if (Build.VERSION.SDK_INT >= 24) {
+        if (getActiveNotification(context)?.all { statusBarNotification -> statusBarNotification.isGroup } == true) {
+            NotificationManagerCompat.from(context).cancelAll()
+        }
+    }
 }
 
 private fun createViewPendingIntent(fcmMessage: MessageFcmMessage, context: Context): PendingIntent {
@@ -119,6 +145,11 @@ private fun getActiveNotification(context: Context, conversationKey: String):
     return Pair(null, null)
 }
 
+
+private fun getActiveNotification(context: Context): Array<StatusBarNotification>? =
+    (context.getSystemService(Context.NOTIFICATION_SERVICE) as NotificationManager?)?.activeNotifications
+
+// TODO: Add a Text saying n messages in m conversations. (this will
 // TODO: Add a Text saying n messages in m conversations. (this will
 // only be visible in API < 24)
 private fun createSummaryNotification(
@@ -210,6 +241,10 @@ private fun updateNotification(
         setGroup(groupKey)
         setContentIntent(createViewPendingIntent(fcmMessage, context))
         setNumber(messageCount)
+
+        val extraData = Bundle()
+        extraData.putInt("zulipMessageId", fcmMessage.zulipMessageId)
+        extras = extraData
     }
 
     val summaryNotification = createSummaryNotification(context, fcmMessage, groupKey)