[FR] update old APIs to use type-backed enums

[DanAlbert]

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Description

We've been doing this for new APIs, but we should go through all the old APIs and make sure that enums have backing types and are used explicitly in function decls.

Aside from improving IDE behavior and ABI checking, this helps languages like Rust: https://github.com/android/ndk/issues/1739#issuecomment-1199911248

[DanAlbert]

Ongoing. Will be for a while. I think there were some improvements between r27 and r28, but we're definitely not done.