Standardizing on Apk as the internal data type means that most of the data
that is needed for the whole lifecycle of a given APK going through this
process will be available regardless of the database changes. Once App
instances are also included, then all of the data should be available
separately from the database. This is important to support parallel
operation. The index could be updated and an app could disappear while an
APK of that app is being downloaded. In that case, it should not show
blank notifications.
Also, in AppDetail, the Apk instance is completely loaded from the db, so
there should not be any nulls on the essential bits like packageName and
download URL.
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