Add Android Bionic support so SwiftNetwork builds on the Android SDK.

[Cartisim]

Motivation

os(Android) is not os(Linux). Swift on Android uses Bionic (import Android), so the existing Glibc / Musl Linux path does not compile as-is.

This change teaches SwiftNetwork that platform with the smallest possible delta: reuse the Linux system layer, import Android where Glibc was required, and paper over the few Bionic ABI differences the compiler actually hits.

Darwin behavior is unchanged.

Modifications

• Enable Logging and SwiftNetworkLinuxShim for .android in Package.swift.
• Compile the Linux system / netlink path as os(Linux) || os(Android).
• Import Android in sources that already imported Glibc.
• Gate Darwin-only files with canImport(Darwin) so they are not built for Android.
• Wrap Bionic’s variadic ioctl as SwiftNetworkLinuxShim_ioctl (unavailable from Swift).
• Match Bionic nullability for recvmsg / sendmsg / if_nametoindex, and use in6_addr.in6_u.u6_addr32 for IPv6.
• Extend the Linux-only #ifs that fail to compile on Android (Logger.notice, errSecSuccess, QUIC session-state callbacks, CryptoKitMetaError).

No new Android-specific networking implementation is added.

Testing

• Built SwiftNetwork for aarch64-unknown-linux-android24.
• Ran the Android test suite. Existing Darwin / Linux interface assumptions (lo0 vs lo, MTU, interface type) still fail on Android and are not addressed here.
• Please also confirm Darwin and Linux still build.

[tfpauly]

Note the overlap with #222

[rpaulo]

@Cartisim could you please let us know if there's anything in this PR that is not in PR 222?

[finagolfin]

@Cartisim, this is strange, that we both submitted this within hours of each other. Since you say you didn't get all the tests passing, while I did in my pull linked above, do you mind if I get mine in first, then you can rebase off that and propose any differences that remain here? Most of our pulls is just adding Android imports, so won't be much left once that's in.

How did you build and test this exactly, natively on Android in the Termux app?