Make automatic platform decision for a given device deterministic

[Deluvi]

Previously, running the -d argument for an android device without explicitly defining the platform would randomly take a platform with the fitting architecture but with a certain API level which was a semi-random choice of all available API levels.

Now, the choice will be to take the default API level of the device architecture, which is currently pointing to API level 21 for all architectures. The user can still specify a particular API using the --platform argument which works with -d.

[kali]

Just a small style issue, I would prefer the helper function to be closer to where it's called, as it is unlikely we will use it anywhere else. If we leave it there, it is likely to drift away from where it is used and it could get confusing: this is one instance where the function body itself is roughly two lines and is more descriptive than the function name...

So, we can shove it in the function body, just before the devices.into_iter() or... inline it as a closure.