Avoids a race condition when dismiss is delayed because the progress bar
hadn't been shown for long enough, while avoiding the flickering that
would be caused when immediately dismissing the dialog earlier.

In case `dismiss` (with delay) is called during an activity and the
activity is consequently destroyed, and calls `dismissNow` in the
process of its cleanup (i.e. in `onDestroy`), the delayed `dismiss`
callback could still try to dismiss the dialog, causing an
`IllegalArgumentException: View[…] not attached to window manager`.

To avoid this faulty behavior, the delayed callbacks are removed
whenever `dismissNow` is called.