decoder: Update check for first mb in slice am: 7e06940dce am: 4fb26568a9 am: 2bf6790f63

Original change: https://googleplex-android-review.googlesource.com/c/platform/external/libavc/+/13171299

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I99b30e4b2ac5f7b0fdeac9adbce15e92d2b36c8f