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

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

Change-Id: I8e6fbf0f95b8b7f5f995a244b1cf22745054c70b