How to set tracker level custom dimension in google analytics android?

Question

How can I set custom dimension on tracker level in android google analytics.

In one of the example it is written on can set user id using following

t.set("&uid", user.getId());

In above code uid is custom dimension or pre defined dimension?

If I want to set few more custom dimensions on tracker level will following code work.

 t.set("&cd3", getCD3());
 t.set("&cd4", getCD4());