s/curVersion/upstreamVersion/g, added suggestedVersion. Refactored QueryBuilder.

In order to support suggested version, I didn't want to have both
suggested version + versionCode in the App table. Rather, just the
code, and then use that (and the apps id) to join onto the apk table.
This is something we wanted to do elsewhere, so I refactored the
QueryBuilder class from the ApkProvider so that it can also be used
by the AppProvider.