[FR] improve the NDK samples

### Description

We want to spend some time improving [the NDK samples](https://github.com/android/ndk-samples), but since we're not app developers and haven't been the owners of that previously, we don't have a great idea of where to start.

The big thing I think we do want to do is improve the documentation throughout the samples. Each sample ought to have a README.md that reads like a tutorial for how to build the sample, and every non-obvious decision in the sample should have a comment explaining _why_ something was done a certain way. In my limited experience, it's not always clear which parts of the sample are essential vs elaboration, or why trade-offs were made (or even where they exist). They also should all be using up-to-date tools (AGP, NDK, etc) and best practices (whatever they may be, like I said I'm not an app developer). Every line of code should either be safe to copy paste into production or should have a comment explaining why it isn't.

But I'm filing this because I suspect that you, the actual app developers, already have a list of things that you wish the samples had done better when you were learning. If so, please share and we'll use that info to guide the fixes :)

(CC @alexcohn, since I think you and I have talked about this in the past but I don't remember specifics)