[flexiglass] FRP support.

When FRP (factory reset protection) is active, System UI shouldn't be
visible, avoiding security holes and letting the setup wizard prompt the
new user of the device to enter the credentials of the previous user.

To achieve this in Flexiglass, we simply observe the
isFactoryResetProtectionActive state and make sure that, if it's true,
the visibility of the entire scene container is false.

Fix: 322197941
Test: manually made sure that SysUI is completely gone and cannot be
interacted with when I hard-coded
DeviceProvisionedController.isFrpActive to return true.
Test: I WASN'T ABLE TO actually trigger FRP to fully test the feature
despite multiple device factory resets
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT

Change-Id: I043507704ccb6ab9f4b828aef74a43c37c21bf53