agent: refactor new battery package (#1872)

agent/battery/battery_stub.go

@@ -0,0 +1,13 @@
+//go:build !darwin && !linux && !windows
+
+package battery
+
+import "errors"
+
+func HasReadableBattery() bool {
+	return false
+}
+
+func GetBatteryStats() (uint8, uint8, error) {
+	return 0, 0, errors.ErrUnsupported
+}