setSemanticsTreeEnabled method

Informs the engine whether the framework is generating a semantics tree.

Only framework knows when semantics tree should be generated. It uses this method to notify the engine whether the framework will generate a semantics tree.

In the case where platforms want to enable semantics, e.g. when assistive technologies are enabled, it notifies framework through onSemanticsEnabledChanged.

After this has been set to true, platforms are expected to prepare for accepting semantics update sent via FlutterView.updateSemantics. When this is set to false, platforms may dispose any resources associated with processing semantics as no further semantics updates will be sent via FlutterView.updateSemantics.

One must call this method with true before sending update through updateSemantics.