Fixes NPE on system app update

When updating a system app, a recent code change resulted in trying to
read the parsed package from a @Nullable field without first checking
for the null case. We have access to the package from the original scan
request and so this change just fetches it from there.

Change-Id: Ieeb4d7f94db23adea64bca7fb982f473cc0a40fe
Fixes: 121411453
Fixes: 121455797
Test: Wait for and observe successful GMS Core update