HLS: faster switching and pause/resume on low buffer

- when upswitching, discard excessive buffering on low
  bandwidth variant, switch to new variant earlier

- when downswitching, report newly found IDR positions
  continuously, and switch as soon as new fetcher passes
  playback position. This allows us to skip time-consuming
  resumeUntil() of old fetcher most of the time

- implement pause/resume on low buffering, and notify
  buffering percentage

- buffering parameter tuning, separate pause/resume/ready
  buffer level and up/down switch buffer level, boost up
  fetcher buffering significantly

bug: 19567254

Change-Id: I750dfcc6f861d78d16a71f501beb86d8129cb048