Mock a /postinstall dir at test time am: e9def4e20e am: 441e236196

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1513842

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: Ifaffb79bdc3ab01bae83f8eb36f0299f97f74739