Add new triggerPostinstall() API to support async postinstall script run am: 586610430e am: 222d2bfac8

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

Change-Id: I49ed88e79053efa98e27b00b70e98d6d0e34cc11
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>