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

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

Change-Id: I3b056ac7e43a3433463476f2de0bb3a42cb76584