Merge "snapuserd: Fix race condition in MergeWorker::WaitForMergeBegin." into main am: c718295a58 am: 9077049910

Original change: https://android-review.googlesource.com/c/platform/system/core/+/2701653

Change-Id: I70266a09d368ec05343bfd957f45706a0cddb777
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>