return type of DisableBatteryOptimization.isBatteryOptimizationDisabled could be tightened to bool instead of bool?

[flodaniel]

Basically the title. Looking at the implementation:

    public static boolean isIgnoringBatteryOptimizations(Context context) {
        if (Build.VERSION.SDK_INT < 23) {
            return true;
        }
        String packageName = context.getApplicationContext().getPackageName();
        PowerManager powerManager = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
        if (powerManager == null) {
            return true;
        }
        return powerManager.isIgnoringBatteryOptimizations(packageName);
    }

the method always returns a boolean, so the flutter implemenation could be changed to:

  static Future<bool> get isBatteryOptimizationDisabled async {
    return await _channel.invokeMethod("isBatteryOptimizationDisabled");
  }