Update PermissionAppsFragment and AppPermissionsFragment.

Show a subtitle with the last time each app used a permission in
PermissionAppsFragment.

Use absolute time instead of relative time.

Add an "allowed only while in use" header to PermissionAppsFragment.
It is only shown when there are elements for it (i.e., only for
location).

Test: Open PermissionAppsFragment.
Change-Id: I4c65d34da60255712c7975fd2559e88fd68b4977