C2AllocatorIon:protect mMappings using mutex am: 416da6e8da am: 670024887e am: d6545589e2 am: 1785465c71

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/av/+/16622627

Change-Id: Ib297e1562da8576b6b52e65a183d2b5f24ca11bb