Update first seek

Created by: nicosang

the goal of this pull request is to simplify the starting of live contents. The main idea is to do the initial seek only after that the first video and audio datas have been pushed : at this moment, the exact timing of video and audio datas are known. The live edge search is still done by the ScheduleController. For each type of datas, the timing of the first request is based on live edge search. Then, in the onBytesAppended callback, the playbackController has to keep in memory the start time for each type of datas. When audio and video datas have been been pushed, the initial seek is done at the earliest time of both type. This avoids misaligned segments.