HLS: temporarily blacklist a variant if failed to fetch

- abort high bandwidth immediately when bandwidth is fluctuating
- use short-term bandwidth estimate for downswitch if bandwidth
  is not stable
- discard bandwidth samples that's too old in absolute time
- if already underflow, switch to lowest bandwidth to catch up
- if error happened during bandwidth switch (likely due to new
  variant link is broken), switch to lowest bandwidth to catch up

bug: 21754330

Change-Id: Ifd16d75e261cefb93b989829bf35a36783142ae0