[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

packages/SystemUI/res/values/flags.xml

@@ -36,4 +36,8 @@
       avatar will no longer show on the lockscreen -->
     <bool name="flag_user_switcher_chip">false</bool>
 
+    <!-- Whether the battery icon is allowed to display a shield when battery life is being
+         protected. -->
+    <bool name="flag_battery_shield_icon">false</bool>
+
 </resources>

packages/SystemUI/src/com/android/systemui/flags/Flags.kt

@@ -222,7 +222,9 @@ object Flags {
     val NEW_STATUS_BAR_WIFI_ICON_BACKEND = unreleasedFlag(609, "new_status_bar_wifi_icon_backend")
 
     // TODO(b/256623670): Tracking Bug
-    @JvmField val BATTERY_SHIELD_ICON = unreleasedFlag(610, "battery_shield_icon")
+    @JvmField
+    val BATTERY_SHIELD_ICON =
+        resourceBooleanFlag(610, R.bool.flag_battery_shield_icon, "battery_shield_icon")
 
     // TODO(b/260881289): Tracking Bug
     val NEW_STATUS_BAR_ICONS_DEBUG_COLORING =