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

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

Change-Id: I62acd8492c2e2761584f2490c8b2db27dcfe3838
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>