Use unsquashfs specified by caller if possible am: f68e825640 am: 96522de5d4

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2624712

Change-Id: I77c2151718483651bbfa60ce9242baab55af8982
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>