Notifications channels are not locked at first boot

After first boot when doing S->T upgrade or when doing factory data
reset on Android T, important system app notification channels are
not greyed out/disabled as expected. DefaultPermissionGrantPolicy
set fixed POST_NOTIFICATIONS are ignored.

If a user then modifies these channels and reboots their device, they
can lock these fixed notification channels into an incorrect state,
where the only recovery is factory reset.

Bug: 254576888
Test: See issue
Change-Id: If73e294b9a732ef47ad685fdd19f51afd8a04eef