Show more shortcuts when last notification is dimissed

We currently only show 2 shortcuts when notifications are
present, but support up to 4 otherwise. With this change,
the hidden shortcuts are added back after dismissing the
notifications, instead of only after closing and reopening
the container.

To ensure the transition is as elegant as possible, we also
separated the shortcuts header from the rest of the shortcuts.
That way we can reveal the new shortcuts without removing the
header (the shortcuts come out from behind the header).

Bug: 38036250
Change-Id: Ie9ab35f9be57cec1d5345e9e70e84e09ea52c9fc