Fix issue #17179314: Make recents limits consistent

The max limit is now 100 (or 50 on svelte devices), and that is
what everyone used.

Re-arranged things so we have a big expensive "fix the world!"
function for recents that we go in to at only select points:
when first initializing the system, when external storage comes
and goes, and if we detect something wrong with the recents
structure.

With that, now getRecentTasks() and addRecentTaskLocked() are
generally much simpler, doing very little work in most cases.
This will help a lot with scaling up to many more recents
entries.

Change-Id: I7b5ae89edc06568f68c8af54a4420aff7635581c