Fix MediaMuxerTest#testWebmOutput failure bug

Change audio/video synchronization method for webm file. Previous method shifts
audio/video separately to 0. This change will get the smallest starting time
among all tracks and shifts them together.
i.e. audio starts at 15 and video starts at 10, old method will shift both of them
to 0 separately, which is not right. New method will shift both of them by 10 (the
smaller of them) where after shifting, audio starts at 5 and video starts at 0.

Bug: 130363039
Test: MediaMuxerTest#testWebmOutput
Change-Id: I97addc72c9e13df649ae65716dc63243dad3849d