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

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

Change-Id: I4a69be5828699eafa29bcf207e711250d51584d4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>