Do not add display area features for untrusted display

Untrusted display (e.g. virtual display created by app, a common
case is screen recorder) doesn't support system decorations. There
is nothing to do with additional features.

This saves 31 layers (according to current features) per untrusted
display. And also reduce the cost of traversing entire hierarchy.

Fixes: 175179353
Test: DisplayAreaPolicyTests#testEmptyFeaturesOnUntrustedDisplay
Change-Id: Iad45217c9e09f33ef9f3f4503b9db2a8a21995a3