Use accessors instead of reaching into the RSA struct am: eedfb29cdb am: 676444f63d am: 1a9a8fe32a

Original change: https://android-review.googlesource.com/c/platform/external/vboot_reference/+/2569310

Change-Id: I97b056a0b9da35dadd3c3360f938b2cc956ba96b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>