Decouple display area from windowing layer

FEATURE_WINDOWED_MAGNIFICATION was a necessary feature of a display.
Because screen level surface animation needs a layer containing
all non-overlay window for reparenting leash. But that seems to
restrict the customizability of non-default display.

This change makes the feature optional by creating an additional
layer if the feature doesn't exist. So a display can create without
any display area features.

Also remove some unused fields.

Bug: 175179353
Test: DisplayContentTests#testValidWindowingLayer
Change-Id: I5c8712e7650a51fc57ea2dc5c60176d7dcafe1d1