Add function to control for which permissions to show usages.

Add a function to control whether or not to show permission usages for
each permission group.  If we do not want to show them, we do not
return the usages or show them in the UI.  We also show a special
string explaining it in AppPermissionFragment and
PermissionAppsFragment.

Note that we currently show usages for all groups, so this is
currently a no-op.

Fixes: 129536503
Test: Temporarily disable a permission's usages and inspect screens.
Change-Id: Ic3482c1cee5d6aac5802be7d761974326f121624