Hide notifications when in Lockdown mode

When the user triggers lockdown the device is put into a secure state,
beyond disabling less secure unlock modalities this should also disable
notifications so that data is not leaked despite the device being
strongly locked.

Fixes: 74564088
Test: Entered lockdown, verified notifications are no longer displayed
on the lockscreen

Change-Id: I4188c36b11a1b0cd496b8032bd246f0413c911c5