Add scheduleEnd to dismiss a Live Activity at a future date

[jenow]

Right now endActivity is the only way to stop an activity, and it dismisses it
immediately. I ran into a case where I wanted to end one but have the system keep
it on the lock screen until a set time and then drop it on its own, even if the
app gets killed in the meantime and no push ever shows up.

ActivityKit already handles this with end(dismissalPolicy: .after(date)), it
just wasn't exposed anywhere, so this wires it up as scheduleEnd:

await liveActivities.scheduleEnd(activityId, at: endDate);

A couple of things about how .after actually behaves that tripped me up:

• the activity leaves the Dynamic Island right away
• you can't update it after that, but a Text(.timer) keeps counting down on the
  lock screen card by itself
• iOS won't keep it around more than 4 hours past the end call

One thing I had to work around: once you schedule the end, the activity is gone
from Activity.activities, so a plain endActivity can't find it to clear it
early. I keep a reference to the ended activity so endActivity can still kill
the card immediately if you need it gone before the timer's up (cancelled
countdown, etc.).

iOS 16.1+. Also added a scheduleEnd stub to the test mock so the platform
interface test still compiles.