Renamed AppTable to AppMetadataTable
See #511 (closed) for details. This is in prep ration for having an even more normalized fdroid_package
table. That table will be the authoritative reference of what "packages" are known about in the client. The "app" table (now "metadata") will be specific to each repository which provides different metadata about that app.
This is a fairly straightforward "Rename Interface" refactoring from Android Studio" and is done so as to minimize the diff in a forthcoming MR.