Add triggerPostinstall API to IUpdateEngine.aidl am: 0aaa736859 am: c840599cbe

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

Change-Id: I4ab5dc976047a493a2671f4f11fd192e9544718b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>