Make toolchain registration optional

[gferon]

The moment you add the current rules_android_ndk to a workspace, the build breaks if ANDROID_NDK_HOME is not set, because native.register_toolchains is always called implicitly.

One solution is to allow users to opt-in into setting up the repository only (which is what you'd do with bzlmod) and either call register_toolchains themselves, or use --extra_toolchains=@androidndk//:all.

Downside: this breaks the current API and we can either use a different approach, or mention it in some release notes

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[gferon]

Superseded by #51