[automerge] C2Allocator: protetct memory mappings from race condition 2p: 5cbca291a6

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/av/+/17806584

Bug: 213850092
Change-Id: Idaba4fbd0f0106a5b964ff9629daf8a8b8612658
Merged-In: Icf6d00491438176e9092b67974a9388e803ac130