[Dock Defend] Update flag to be resource-backed so we can enable it on

certain devices only.

Fixes: 261156040
Bug: 255625888
Test: `adb shell am broadcast -a com.android.systemui.demo
    -e command battery -e overheated true` -> shield displays on titan
    but doesn't display on other devices
Change-Id: I40c0d841ba0c8e1c41bbdb13abbaf0c0ef5125af