lastcron indicator is not using scheduled tasks

On one of our sites we have disabled all activity modules.

Unfortunately that causes the 'cron not run recently' warning to occur as it does:

$lastcron = $DB->get_field_sql('SELECT MAX(lastcron) FROM {modules}');

Perhaps we could use scheduled tasks lastrun for this indicator instead.

Steps to reproduce:

1. Install a new Moodle dont setup cron
2. Disable all activity modules
3. Run cron manally

Expected result:
The 'cron.php maintenance script</a> has not been run for at least 24 hours.' does not appear.

Actual result:
The cron warning appears and never disappears.