Don't pause Animator when lifecycle state paused

[ColdPaleLight]

Fix issue:
flutter/flutter#45131
flutter/flutter#91717

Detail: flutter/flutter#45131 (comment)

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[dnfield]

This seems like it means we'll call Dart_NotifyIdle more often, right? In that case, we could end up doing more GC work than we really ought to be when the application is backgrounded.

[ColdPaleLight]

This seems like it means we'll call Dart_NotifyIdle more often, right? In that case, we could end up doing more GC work than we really ought to be when the application is backgrounded.

@dnfield
When the app is in the background, AppLifecycleState is AppLifecycleState.paused. So the SchedulerBinding._framesEnabled has been set to false.
https://github.com/flutter/flutter/blob/ab0a33597328772fe27dbc0b217ac930373ef5f0/packages/flutter/lib/src/scheduler/binding.dart#L348-L361

  void handleAppLifecycleStateChanged(AppLifecycleState state) {
    assert(state != null);
    _lifecycleState = state;
    switch (state) {
      case AppLifecycleState.resumed:
      case AppLifecycleState.inactive:
        _setFramesEnabledState(true);
        break;
      case AppLifecycleState.paused:
      case AppLifecycleState.detached:
        _setFramesEnabledState(false);
        break;
    }
  }

Therefore, after the App enters the background,SchedulerBinding.scheduleFrame will return early here. So Animator::RequestFrame and Dart_NotifyIdle will not be called more often than before.
https://github.com/flutter/flutter/blob/ab0a33597328772fe27dbc0b217ac930373ef5f0/packages/flutter/lib/src/scheduler/binding.dart#L781-L783

  void scheduleFrame() {
    if (_hasScheduledFrame || !framesEnabled)
      return;

This PR is mainly to allow scheduleForcedFrame to work correct in the future.
https://github.com/flutter/flutter/blob/ab0a33597328772fe27dbc0b217ac930373ef5f0/packages/flutter/lib/src/scheduler/binding.dart#L814-L818

  void scheduleForcedFrame() {
    // TODO(chunhtai): Removes the if case once the issue is fixed
    // https://github.com/flutter/flutter/issues/45131
    if (!framesEnabled)
      return;

Simplifying the logic of Animator::RequestFrame will also benefit the maintenance of the engine.

[dnfield]

I'm a big fan of this in that case.

I'd appreciate input from @chinmaygarde and @iskakaushik (and of course @gaaclarke and @chunhtai if they have any input)

[chinmaygarde]

Do we still need this since the framework ought to handle frame scheduling now. If it is redundant, we can get rid of the RunNowOrPostTask call completely. We can do this in a later patch if necessary.

[dnfield]

(@ColdPaleLight feel free to add the waiting for tree to go green label if you think this is good to go, I'm holding off on adding it in case you plan to address this now)

[ColdPaleLight]

I'll do this in a later patch.