Rework first device encounter

Instead of relying on IDeviceListerCb::onDeviceMounted to return true to denote the first encounter with a device, delegate that job to a dedicated IMediaLibrary method.

We can't rely on IDeviceListerCb anymore since the device listers will only be started during the discovery, and the media library user might want to know if the device is seen for the first time to know whether they should index a removable device or not, so before the initial discovery happens.