ndk: Replace open-coded `bool` getters with `bitflags` wrappers

[MarijnS95]

And "correct" their ABI to match how it is used in the actual NDK getter functions. Also add missing aliases and upstream documentation where sensible.

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CC @rib, I've been thinking about this to make our wrapper types more consistent, but it may be more inconvenient to use now except in the case when checking multiple flags at once...

[MarijnS95]

We should also do this for ndk::configuration::DiffResult, which most likely benefits from the autogenerated Debug implementation too.