Ensure toast is shown on error condition.

AppPermissionUsageFragment tries to show a toast when it cannot find
the given app.  However, the activity does not finish immediately and
instead crashes when the app info is null, stopping the toast from
appearing.  By correctly avoiding that crash, we ensure the toast is
shown.

Change-Id: I881f696dab5ee1c61f95265cbfc1e9c5ecfe71bd
Fixes: 123581716
Test: Load page with illegal app name and see toast.