Allow notifications to not specify a contentIntent.

If they don't, the click events will be passed through to the individual
views in the notification view, which may have their own PendingIntents
attached.

Previously, it was against the UX spec to allow this, but now we are
changing that and will have buttons in there.

Change-Id: I674234212f64b2b8802a0708b7eed0614e147ca3