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

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

Change-Id: I70b699507a5d0f540b71037e2005d3b7219d85b2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>