Media.Duration UMA reporting can occur more than once per playback (at least for MSE)

This bug tracks getting this fixed.
renganathan@ and I are spinning up an internal doc to discuss the changes.

TLDR, ATM, We're thinking, very roughly:
1) split out src= from MSE duration UMA reporting (and maybe not do it for MSE, since that can change multiple times during lifetime of the player).
2) for MSE (and maybe also src=) report playback duration of some kind (not necessarily buffered media duration)
3) for MSE, maybe report duration on each "endOfStream()" call + also the number of endOfStream() calls per actual playback? (Will UMA allow us to normalize these??)