better mpeg2 TS elementary stream Access Unit parsing

mpeg2 es stream access units have a 3 byte prefix and a 1 byte start
code. Searching for the next access unit started after the prefix
instead of after the start byte.

Bug: 74114680
Test: ran POC before/after
(cherry picked from commit 371066d0)

Change-Id: I813d2ecd3bc972fb92809f1068f596970591cf3c