Allow notifications to be smaller in width than its container.

Add a width dimension that the children of the
NotificationStackScrollLayout can use to make a separate MeasureSpec for
its children.

Also, adjust the translation of the row so that the row can be swiped
off screen without any clipping if the width is smaller than the
notification panel itself.

Test: booted on phone and Android Auto headunit
Bug: 36692077
Change-Id: Ic8bb8a707c4d91f4e38d5ee3461b406bf14d0042