Keep DisplayAreaGroup and DisplayAreaPolicyBuilder in services.jar

After aosp/2123992, this 2 classes are not visible by
CarDisplayAreaPolicyProvider, result in boot loop.

Use @Keep to keep them in services.jar solve the booting issue.

Fix: 239744560
Test: Manually luanch seahawk_car_ui_portrait
Change-Id: Ie4c934b7c729017e2bd0ff8cf99d059cf9275da3