JavaScriptKit: import Android module for non-Wasm Android targets

[ken0nek]

Summary

Adds a canImport(Android) branch to ThreadLocal.swift's platform-import conditional so JavaScriptKit compiles for the Swift 6.3 Android SDK.

Problem

Building any JavaScriptKit consumer for an Android triple via the official Swift SDK for Android (swift-6.3.1-RELEASE_android) currently fails in Sources/JavaScriptKit/ThreadLocal.swift:

error: cannot find 'pthread_key_create' in scope
    pthread_key_create(&key, nil)
    ^
error: cannot find 'pthread_key_t' in scope
    var key = pthread_key_t()
              ^
error: 'nil' requires a contextual type

Android uses Bionic, which Swift exposes as the Android module — not Glibc. The existing conditional therefore falls through to #error("Unsupported platform") on Android triples.

Reproduction

swift sdk install https://download.swift.org/swift-6.3.1-release/android-sdk/swift-6.3.1-RELEASE/swift-6.3.1-RELEASE_android.artifactbundle.tar.gz \
    --checksum 8193a4e96538635131a154736c8896fba0e5a1c30e065524f00ed78719bac35a

# From any package that depends on JavaScriptKit:
swift build --swift-sdk swift-6.3.1-RELEASE_android

Fix

Insert a canImport(Android) branch between the Darwin and Glibc branches, matching the pattern used in Foundation (swift-corelibs-foundation) and swift-corelibs-libdispatch for the same pthread imports:

#elseif canImport(Darwin)
import Darwin
#elseif canImport(Android)
import Android
#elseif canImport(Glibc)
import Glibc

ThreadLocal.swift is the only file under Sources/JavaScriptKit/ that has this conditional-import shape; all other Swift sources either are Wasm-gated already or don't reference libc. No behavior change on Wasm, Darwin, or Glibc targets — only the previously-unreachable Android branch is newly covered.

Test plan

• swift build --swift-sdk swift-6.3.1-RELEASE_android succeeds through the Swift-compile phase for a consumer package that uses @JS (tested on x86_64-unknown-linux-android30). Without the patch: fails with the errors above. With the patch: compiles cleanly.
• swift package --swift-sdk swift-6.3.1-RELEASE_wasm js still produces byte-identical Wasm output for the same consumer (no Wasm regression).
• Darwin / Glibc builds — unchanged by the patch; existing CI covers these paths.

Context

Part of enabling Swift-on-Android via the Swift 6.3 Android SDK + upcoming @c attribute for ABI-stable C exports from @JS classes. Downstream work (separate from this PR) consumes JavaScriptKit on Android through the Android .so + JSI path on React Native; that work is blocked only by this one import conditional.