You cannot instantiate this class. One can use the getters/setters when using it in the context of QuickAction.QuickActionDefaultsHandler.
The following are methods for SendEmailQuickActionDefaults.
public void setIgnoreTemplateSubject(Boolean useOriginalSubject)
Type: void
public void setInsertTemplateBody(Boolean keepOriginalBodyContent)
Type: void
public void setTemplateId(Id templateId)
Type: void