Adjust order of calculation to prevent integer underflow am: 8929a8ef29 am: 0c87bce93e am: bf0219683e am: 768332732c

Original change: https://android-review.googlesource.com/c/platform/test/vts-testcase/security/+/1953491

Change-Id: I49443a9fc54e53a32f0761d5f7ebe7199882c0fc