Add stubs for new API triggerPostinstall am: 9c5baeb774 am: 171f0cba64

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3335898

Change-Id: If027b4f7766a085ad1035c95cbbbeca419e83248
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>