Refactor HLS getStartTime to avoid unnecessary requests
Refactor HLS getStartTime to avoid unnecessary requests

In the hls parser's |getStartTime_|, we make network requests to
to determine a stream's start time.

To make the code easier to understand, this CL changes the code to
makes requests differently for each codec, to:
  - allow mp3 to make no requests
  - clarify mp4 to make 1 or 2 requests

Issue #1558

Change-Id: I7c83f5e326cd5408ff33576a958aee2f2b50fb60

tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player