Add flag com.android.providers.media.flags.enable_backup_and_restore to trunk_staging.

This change was automatically created by advancing the Gantry workflow at:
https://releasetracker.partner.android.com/changes/6a3408i000001?build_unique_id=android_platform_mediaprovider
Do not edit this file manually or revert this change - use the Gantry workflow to manage the state of this flag.

Bug: 195138692
Flag: com.android.providers.media.flags.enable_backup_and_restore
Ignore-AOSP-First: AConfig change for future AOSP release
Change-Id: I92da2068f017749966599e99ff798238a50443c8