по умолчанию стоит true уже
Whether or not the activity can be instantiated by the system — "true" if it can be, and "false" if not. The default value is "true".
The <application> element has its own enabled attribute that applies to all application components, including activities. The <application> and <activity> attributes must both be "true" (as they both are by default) for the system to be able to instantiate the activity. If either is "false", it cannot be instantiated