HLS: misc bug fixes

- fix no target-duration case

- fix for audio-only <=> audio/video switching

- disable audio-only variants if there is at least
  one variant with video

- fix mpeg2ts PTS wraparound when bandwidth adapting

- tweak up/down switch marks

bug: 19567254

Change-Id: Ib46144203c56dfc96eccd6ddaa3867e8a4f2c6a9