Change TOP_TO_FGS_GRACE_DURATION to be DeviceConfig

It was still a global setting.

Bug: n/a
Test: Manual test using the following commands
- `dumpsys activity settings | grep -w top_to_fgs_grace_duration`
- `device_config put activity_manager top_to_fgs_grace_duration 5000`
- also adb reboot and make sure it's persisted

Change-Id: I5d058bc7a6845e4c154c5bacef5489b0be512d68