delay call to setState with addPostFrameCallback

[atavism]

For https://github.com/getlantern/engineering/issues/1652

══╡ EXCEPTION CAUGHT BY FOUNDATION LIBRARY ╞════════════════════════════════════════════════════════
The following assertion was thrown while dispatching notifications for ValueNotifier<bool>:
setState() or markNeedsBuild() called during build.
This VPNSwitch widget cannot be marked as needing to build because the framework is already in the
process of building widgets. A widget can be marked as needing to be built during the build phase
only if one of its ancestors is currently building. This exception is allowed because the framework
builds parent widgets before children, which means a dirty descendant will always be built.
Otherwise, the framework might not visit this widget during this build phase.
The widget on which setState() or markNeedsBuild() was called was:
  VPNSwitch
The widget which was currently being built when the offending call was made was:
  FfiValueBuilder<String>

When the exception was thrown, this was the stack:
#0      Element.markNeedsBuild.<anonymous closure> (package:flutter/src/widgets/framework.dart:5177:9)
#1      Element.markNeedsBuild (package:flutter/src/widgets/framework.dart:5189:6)
#2      State.setState (package:flutter/src/widgets/framework.dart:1223:15)
#3      _VPNSwitchState.build.<anonymous closure>.<anonymous closure> (package:lantern/vpn/vpn_switch.dart:118:15)
#4      _AdvancedSwitchState._handleControllerValueChanged (package:flutter_advanced_switch/flutter_advanced_switch.dart:279:23)
#5      ChangeNotifier.notifyListeners (package:flutter/src/foundation/change_notifier.dart:437:24)
#6      ValueNotifier.value= (package:flutter/src/foundation/change_notifier.dart:559:5)
#7      _AdvancedSwitchState.didUpdateWidget (package:flutter_advanced_switch/flutter_advanced_switch.dart:108:24)
#8      StatefulElement.update (package:flutter/src/widgets/framework.dart:5789:55)
#9      Element.updateChild (package:flutter/src/widgets/framework.dart:3941:15)
#10     ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:5642:16)
#11     StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:5780:11)
#12     Element.rebuild (package:flutter/src/widgets/framework.dart:5333:7)
#13     BuildScope._tryRebuild (package:flutter/src/widgets/framework.dart:2693:15)
#14     BuildScope._flushDirtyElements (package:flutter/src/widgets/framework.dart:2752:11)
#15     BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:3048:18)
#16     WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:1162:21)
#17     RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:468:5)
#18     SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1397:15)
#19     SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1318:9)
#20     SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:1176:5)
#21     _invoke (dart:ui/hooks.dart:312:13)
#22     PlatformDispatcher._drawFrame (dart:ui/platform_dispatcher.dart:419:5)
#23     _drawFrame (dart:ui/hooks.dart:283:31)

[jigar-f]

LGTM