change-spl: Detect image format before simg2img

This commit reads the sparse image magic word before calling simg2img
so that `change_security_patch_var.sh` can support both sparse and
unsparse system images.

Bug: 79395599
Test: ./change_security_patch_ver.sh with the latest aosp_arm64 image
Change-Id: I207195fa9a3922aa24cca7317dbfc0aaf2f64a36
Merged-In: I207195fa9a3922aa24cca7317dbfc0aaf2f64a36