Check for public key before checking for signatures am: f624e1167d am: 9e14d49301 am: 0db5963bd8 am: 02118a7841

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1428103

Change-Id: Iebb7a68fb454d86bbd4377e07bb10e97c368485e