ci: add Android cross-compilation CI job

Add CI job that cross-compiles libbitcoinkernel-sys for three Android
targets: aarch64-linux-android, armv7-linux-androideabi, and
x86_64-linux-android.

Author: jaoleal

.github/workflows/ci.yml

@@ -211,6 +211,45 @@ jobs:
     - name: Build and test
       run: cargo test -vv
 
+
+  android-cross-compile:
+    name: Cross-Compile for Android (${{ matrix.target }})
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        target:
+          - aarch64-linux-android
+          - armv7-linux-androideabi
+          - x86_64-linux-android
+    steps:
+    - uses: actions/checkout@v4
+
+    - name: Install Rust toolchain
+      uses: dtolnay/rust-toolchain@stable
+      with:
+        targets: ${{ matrix.target }}
+
+    - name: Cache dependencies
+      uses: Swatinem/rust-cache@v2
+
+
+    - name: Install Boost library
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y libboost-all-dev
+
+    - name: Set up Android NDK
+      uses: nttld/setup-ndk@v1
+      id: setup-ndk
+      with:
+        ndk-version: r27c
+
+    - name: Build
+      env:
+        ANDROID_NDK_HOME: ${{ steps.setup-ndk.outputs.ndk-path }}
+      run: cargo build -p libbitcoinkernel-sys --target ${{ matrix.target }} -vv
+
   fuzz-corpus:
     name: Verify Fuzz Corpus
     runs-on: ubuntu-latest