Show usage duration in Permissions Hub.

In addition to showing the number of accesses we can also show
duration (for permissions where that makes sense).

I also fixed the strings to correctly say "access" instead of
"accesses" when there is only one access.

Test: Open Permissions Hub, see singular and plural strings with and
without duration.

Change-Id: I9330a257a06eef0c9ec3fe0bdc63e724956afd57