Implement trigger postinstall API am: 44bcf1fd69 am: 8fd1b01793

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

Change-Id: I14fc4a55f5c29ac4379dff6e16deefb9cba26501
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>