Implement trigger postinstall API am: 44bcf1fd69

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3391043

Change-Id: I51ace235672f3f58c7829937c9dcbe8112bb4c34
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>