C2AllocatorIon:protect mMappings using mutex am: 416da6e8da am: 670024887e am: d6545589e2 am: 1785465c71 am: aef7c3d8e0

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

Change-Id: Ied49bd06a08f49a8486faf5b7766460953012826