Add triggerPostinstall API to IUpdateEngine.aidl am: 0aaa736859

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3350135

Change-Id: If6ff9a237809ef1735c6e1dedb967a808a5fe97a
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>