[DOC] add version script usage to best practices doc

[DanAlbert]

Using a version script that restricts your library to exposing only your JNI interface is always a good idea and helps avoid the annoying unwinder issues. We should probably recommend these by default (and maybe update the samples and Studio project templates?)

[DanAlbert]

https://developer.android.com/ndk/guides/middleware-vendors covers this for the AAR case. It's slowly working its way into the samples as I rewrite and touch up the old ones. It's not in the Studio templates yet but is in https://github.com/danalbert/ndk-app-template which is a sort of prototype for what those templates ought to be.

I've sent a doc for review that covers the concept in general on DAC. Will link that here when it's live.

[DanAlbert]

https://developer.android.com/ndk/guides/symbol-visibility