In Dragon Ball, aura always meant ki. Recently, however, fighters without ki have been shown to have an aura. First 17, which now it's been explicitly stated he doesn't have ki. Kochiarator, which is a fusion of 3 robots, has been shown to have a sort of aura. And Anisa which is a fusion of 3 robots with a humanoid form definitely has an aura that isn't distinguishable from any other fighter's regular ki aura. So if 17's and other robots' aura aren't ki, what are they supposed to be?
What is ki?
It's a form of life force energy used by characters in the Dragon Ball Universe.
We consider the aura to be a form of ki and yes it is! Rather than ki, you should look at it as energy. They might both sound very similar but look at it this way. The ki blast fired by a character is different from the ki used while flying. So the aura has to do more about powering up and using energy. A good way to look at the same is if Android 17 were to have unlimited ki in terms of strength, he would technically be stronger than even the Grand Priest because of his unlimited strength, however, the androids only have unlimited stamina(life-energy source).Think of Android 17's power up to him using all his strength.
We know all living beings in Dragon Ball have ki. We also know that the amount of ki can be increased or by training and altered by transformations or special skills like the Kaio-Ken. We also know that this Ki is not unlimited irrespective of who the being is. There have been many instances where a character (Didn't have enough energy to fight) or didn't have sufficient amount of energy to power up etc which is why they consume Senzu beans.
Androids, on the other hand, have a special or unnatural energy source such that their ki capacity is unlimited. Unlike the other species(All species shown on Dragon Ball as of now). Due to the same, androids also have difficulty in sensing ki the way other species do and need special modifications to help locate a particular fighter.
There is not definite available answer but this is what I believe. I think the amount of ki associated with stamina and energy has nothing to do with one's strength. In other words, when Goku uses his Super Saiyan transformations and alters his ki, I don't think he gains more life support or more stamina(as a matter of fact, he loses more stamina), yet his ki is altered. The same applies with regard to the androids. The only difference is, the androids have an unlimited source of life-energy(Stamina associated ki). We know Android 17 and even 18 obviously have gotten a lot stronger since the android saga. If they had a fixed capacity of strength ki, they wouldn't have been able to get stronger.
Also, Koicherator, Aniraza, and the Androids shouldn't be confused with.