reuse DEFAULT_PRESENTATION_POD as a const

...cala/org/bigbluebutton/core/models/PresentationPods.scala

...c/org/bigbluebutton/modules/polling/model/PollingModel.as

...g/bigbluebutton/modules/polling/service/PollingService.as

.../bigbluebutton/modules/present/managers/PresentManager.as

...luebutton/modules/present/model/PresentationPodManager.as

...luebutton/modules/present/services/PresentationService.as

...ton/modules/present/services/messaging/MessageReceiver.as

...uebutton/modules/present/ui/views/PresentationWindow.mxml

...org/bigbluebutton/modules/present/ui/views/SlideView.mxml