Bug 1673716 - Move the AboutHomeStartupCache shutdown blocker timeout to the onShutdown method. r=emalysz

In an earlier attempt to fix this shutdown hang, a timeout was added to the cacheNow
task function to try to have a maximum of 1s of wait time during the shutdown blocker
before giving up and letting the shutdown proceed.

This didn't seem to put a dent in the shutdown hangs. It looks like DeferredTasks
that are being finalized don't actually re-enter the task if the task was already
running, which might explain why in some cases the timeout wasn't being hit. This
patch makes sure that the timeout is being used regardless of whether or not the
cache task is already underway.

Differential Revision: https://phabricator.services.mozilla.com/D98688