Rather, only show 25 progress events. I went with "25 events" rather
than "every X apps" because then it will be nice for both large repos
(e.g. F-Droid will update every ~50 apps) and small repos (something
with 20 apps will update for every one, but not add much overhead).

On my testing with an API 11 emulator here, it went from ~32s to
process ~1100 apps to ~20s. When no progress events are sent, then it
also takes ~20s, so this essentially is a 50% improvement for this part
of the update process.