Add stubs for new API triggerPostinstall am: 9c5baeb774

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

Change-Id: Ic5738c605629913e7eca6fe25a000e57dba3e18c
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>