Wait for process to exit before unmounting postinstall partition am: 1e926315ee am: b1e5ee3dfd am: b5f0e3dd50 am: 6cc629b7be

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2767665

Change-Id: I0528022351ef02c9a58f6d4b9a9a68ef115725f4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>