Use unsquashfs specified by caller if possible am: f68e825640

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

Change-Id: I2bd88d90fa5f2326245f50aa0ce3ab1cbce7b308
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>