How do i dynamically choose which activity to launch when opening an app

No, since you have to run some code, there’s no way to declaratively (in manifest) to say this. You have to launch an activity (set in manifest), then have this activity decide based on if the user is logged on or not what second activity to launch via Intent:

final Class<? extends Activity> activityClass;
if(userIsLoggedOn())
    activityClass = LoggedOnActivity.class;
else
    activityClass = LogInActivity.class;

Intent newActivity = new Intent(context, activityClass);
context.startActivity(newActivity);