Also support "long" app sizes when parsing JSON

The DownloadFileTask already supports "long" for the total file size, so
we should do so here, too.
Fix the timestamp to be parsed as "long" instead of "int"
Initially show the size as unknown for external builds of size 0
Remove a redundant variable initialization in DownloadFileTask
Generalize DownloadFileTask a bit so it can be more easily extended
Add a GetFileSizeTask to determine the file size of externally hosted APKs