An incoming request is more than a tab now — it can open an editor, compose a
timer or ask which alarm to dismiss. So it is read once, when the activity is
created rather than on every configuration change: re-deriving it on a rotation
would drag the user back into an editor they had just left.
The extras are unparcelled behind a guard here too. `MainActivity` is exported,
and before this it only ever read the action, which never unparcels.