getThumbnail
Called to get thumbnail bitmap. This method is called on UI thread. When provider finds cache bitmap, it may invoke ResultCallback.onThumbnailLoaded immediately. Provider may start background thread and invoke ResultCallback.onThumbnailLoaded later in UI thread. The method might be called multiple times for the same position, PlaybackSeekDataProvider must guarantee to replace pending ResultCallback with the new one. When seeking right, getThumbnail() will be called with increasing index; when seeking left, getThumbnail() will be called with decreasing index. The increment of index can be used by subclass to determine prefetch direction.
Parameters
Index of position in .getSeekPositions.
The callback to receive the result on UI thread. It may be called within getThumbnail() if hit cache directly.