Add flag com.android.window.flags.relative_insets to trunk_staging.

This change was automatically created by advancing the Gantry workflow at:
https://releasetracker.partner.android.com/changes/42p1kka000001?build_unique_id=android_platform_windowing_frontend
Do not edit this file manually or revert this change - use the Gantry workflow to manage the state of this flag.

Bug: 277292497
Flag: com.android.window.flags.relative_insets
Ignore-AOSP-First: AConfig change for future AOSP release
Change-Id: If6dd4398dcac9d29411366d41b6575b92ada2fcc