Expose presentation start time
Expose presentation start time

Closes #957

Change-Id: I707fcafde6ad44eac6605390d263671f914a045b

TheModMaker/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,TheModMaker/shaka-player,tvoli/shaka-player,TheModMaker/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,vimond/shaka-player,TheModMaker/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,vimond/shaka-player,shaka-project/shaka-player,vimond/shaka-player