Add Android Swift SDK cross-build CI job

Adds a Linux-container job that installs the official
swift-6.3.2-RELEASE_android Swift SDK, sets up the Android NDK
(r27d) via the SDK's setup-android-sdk.sh script, and cross-compiles
for aarch64-unknown-linux-android28 and x86_64-unknown-linux-android28
using swift build --swift-sdk <triple> --static-swift-stdlib, as
documented at swift.org/documentation/articles/swift-sdk-for-android-getting-started.html.
This complements the existing Skip-toolchain-based Android job.

Author: colemancda

.github/workflows/swift.yml

@@ -55,3 +55,41 @@ jobs:
               brew install skiptools/skip/skip || (brew update && brew install skiptools/skip/skip)
               skip android sdk install --version ${{ matrix.swift }}
               ANDROID_NDK_ROOT="" skip android build --arch ${{ matrix.arch }}
+
+  android-sdk:
+      name: Android (Swift SDK, ${{ matrix.triple }})
+      runs-on: ubuntu-latest
+      container: swift:6.3.2
+      timeout-minutes: 30
+      strategy:
+        fail-fast: false
+        matrix:
+          triple: ["aarch64-unknown-linux-android28", "x86_64-unknown-linux-android28"]
+      steps:
+        - name: Checkout
+          uses: actions/checkout@v4
+        - name: Swift Version
+          run: swift --version
+        - name: Install dependencies
+          run: apt-get update -y && apt-get install -y curl unzip
+        - name: Install Android Swift SDK
+          run: |
+            set -eux
+            url="https://download.swift.org/swift-6.3.2-release/android-sdk/swift-6.3.2-RELEASE/swift-6.3.2-RELEASE_android.artifactbundle.tar.gz"
+            curl -fsSL "$url" -o android.artifactbundle.tar.gz
+            swift sdk install android.artifactbundle.tar.gz
+            swift sdk list
+        - name: Install and configure the Android NDK
+          run: |
+            set -eux
+            unset ANDROID_NDK_ROOT
+            cd ~/.swiftpm/swift-sdks/swift-6.3.2-RELEASE_android.artifactbundle/swift-android/
+            curl -fSL -o ndk.zip "https://dl.google.com/android/repository/android-ndk-r27d-Linux.zip"
+            unzip -qo ndk.zip
+            echo "ANDROID_NDK_HOME=$PWD/android-ndk-r27d" >> "$GITHUB_ENV"
+            ANDROID_NDK_HOME="$PWD/android-ndk-r27d" ./scripts/setup-android-sdk.sh
+        - name: Build
+          run: |
+            set -eux
+            unset ANDROID_NDK_ROOT
+            swift build --swift-sdk ${{ matrix.triple }} --static-swift-stdlib