Test app improvements for seek and close caption

[emdobrin]

Task description

Improve test apps for Media for Edge Network extension with examples for:

• seek detection in video player
• closedCaption button in video player. Also update CC related deprecated APIs - see https://developer.apple.com/documentation/avfoundation/avplayer/1386458-closedcaptiondisplayenabled
• if possible, also add some subtitles.

Additional implementation details or code snippet(s)

func detectCCChange() {
    guard let currentItem = self.player.currentItem else { return }
    let asset = currentItem.asset

    if #available(iOS 15.0, *) {
      asset.loadMediaSelectionGroup(for: AVMediaCharacteristic.legible, completionHandler: { group, _ in
        guard let group = group else { return }
        self.detectCCStatus(item: currentItem, group: group)
      })
    } else {
      guard let group = asset.mediaSelectionGroup(forMediaCharacteristic: AVMediaCharacteristic.legible) else { return }
      detectCCStatus(item: currentItem, group: group)
    }
  }

  func detectCCStatus(item: AVPlayerItem, group: AVMediaSelectionGroup) {
    let option = item.currentMediaSelection.selectedMediaOption(in: group)
    let ccActive = (option != nil)
    if self._isCCActive != ccActive {
      self._isCCActive = ccActive
      var info: [String: Any] = [:]
      info["ccActive"] = self._isCCActive
      NotificationCenter.default.post(name: NSNotification.Name(rawValue: PlayerEvent.PLAYER_EVENT_CC_CHANGE), object: self, userInfo: info)
    }
  }func detectCCChange() {
    guard let currentItem = self.player.currentItem else { return }
    let asset = currentItem.asset

    if #available(iOS 15.0, *) {
      asset.loadMediaSelectionGroup(for: AVMediaCharacteristic.legible, completionHandler: { group, _ in
        guard let group = group else { return }
        self.detectCCStatus(item: currentItem, group: group)
      })
    } else {
      guard let group = asset.mediaSelectionGroup(forMediaCharacteristic: AVMediaCharacteristic.legible) else { return }
      detectCCStatus(item: currentItem, group: group)
    }
  }

  func detectCCStatus(item: AVPlayerItem, group: AVMediaSelectionGroup) {
    let option = item.currentMediaSelection.selectedMediaOption(in: group)
    let ccActive = (option != nil)
    if self._isCCActive != ccActive {
      self._isCCActive = ccActive
      var info: [String: Any] = [:]
      info["ccActive"] = self._isCCActive
      NotificationCenter.default.post(name: NSNotification.Name(rawValue: PlayerEvent.PLAYER_EVENT_CC_CHANGE), object: self, userInfo: info)
    }
  }