decoder: Update check for first mb in slice am: 7e06940dce am: 4fb26568a9 am: 2bf6790f63 am: 5f4f6e2ecf am: 640eda280b

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

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Ia0b4bb57e604986789246f960db871c3178ebed3