Add new triggerPostinstall() API to support async postinstall script run am: 586610430e

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/3335897

Change-Id: I0576a478df6b2a373ac9cf0875543b4602da2ca3
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>