Fix subtitle buffered check

[robwalch]

This PR will...

Fix an issue where BufferHelper.bufferInfo throws when it is passed an array of TimeRange objects from the subtitle-stream-controller.

Why is this Pull Request needed?

BufferHelper.bufferInfo and getBuffered expect an HTMLMediaElement or SourceBuffer (or something implementing the Bufferable interface) and not an array of TimeRanges, which subtitle-stream-controller was setting on mediaBuffer.

The stronger typing on media, mediaBuffer, and videoBuffer informed all of the changes in this PR, which will prevent exceptions when checking if subtitles are buffered while seeking or when the player is detached in certain event loops.

While the exception in BufferHelper.bufferInfo is caught, it can be reproduced by enabling "Pause on caught exceptions" and seeking outside the buffered range on any HLS stream with an active subtitle track and playlist. A result is that without an accurate reading on the buffered range for sub, active requests would not be aborted when seeking to prioritize newer requests like they are for audio and video.

Checklist

• changes have been done against master branch, and PR does not conflict