HLS: some fixes for seek with discontinuity.

- keep old fetcher when seeking, unless the URI is changing.

- when restarting after a seek, check discontinuity seq, and
  queue format change if it's changed.

- add a simple kill switch to abort when stop (or pause for seek).

- when seeking, if searching for start time goes into 2nd segment,
  do not signal time discontinuity or reset first PTS.

- use setFormat() to set format in AnotherPacketSource, otherwise
  video/audio flags are not updated and format are not cleared on
  discontinuities.

- do not start queueing video access unit until first IDR after start

bug: 19656539

Change-Id: I79108d26964f59ea00d2eeac8f5f9318747f8541