Add 'Mark all as read' button to overlay

Button is only shown if there are more unread notifications than
fit in the overlay.

To avoid performance issues, this version only works for cases where
the number of unread notifications is less than the maximum count
(99 currently). Otherwise the button to mark all as read isn't
displayed (it's also limited on the server-side for good measure).

Bug: 47092
Change-Id: Ifcb0a436e2b31062741c441cca239d35ddefa0e1
Merge "Add 'Mark all as read' button to overlay"