Don't show notification shade on keyguard when disabled.

When StatusBarManager.DISABLE2_NOTIFICATION_SHADE is set, hide the
notification shade from the lock screen. This is mainly used for
LockTask mode, where the device policy client (DPC) app can choose to
enable the keyguard while suppressing all notifications.

Note that this applies to the lock screen even when the device isn't
protected by a PIN/pattern/password.

Bug: 68367551
Test: runtest systemui
Test: CTS verifier > Managed provisioning > Device owner tests
      > LockTask UI
Change-Id: Id2c37aa245582822bdc517b292344e3f83c268ea