Cleanup PayloadSigner::AddSignatureToManifest() am: 7010853f44 am: e25fecb585 am: 43a27827b8

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/1500982

Change-Id: Ica4c08728fcee5c1d0f8f1ea9848de17fff7fd86