Add Android to the CI build matrix (#2165)

Support building for Android on Arm and AArch64 platforms.

Modify the build matrix to add to new variations. Both variants
download, extract and setup the Android Native Development Kit (NDK) on
a Linux runner. Each variant specifies a `android_arch_abi`, which is
passed to CMake during configuration as its `CMAKE_ANDROID_ARCH_ABI`
option.

The CMake toolchain file provided by the NDK is used when building for
Android. The NDK version used is r27c, which is the latest Long-Term
Support (LTS) version. `ANDROID_PLATFORM` is intentionally not set, so
the NDK can default to the minimum supported version, which is 21. The
compiler (Clang) version used by this NDK is 18.0.3.

The NDK ships with its own sysroot, which has the Linux kernel headers
of version 6.8.0, or `LINUX_VERSION_CODE 395264`.

CMake
https://cmake.org/cmake/help/latest/manual/cmake-toolchains.7.html#id23
https://cmake.org/cmake/help/latest/variable/CMAKE_ANDROID_ARCH_ABI.html

NDK
https://developer.android.com/ndk/downloads

.github/workflows/presubmit.yml

@@ -27,6 +27,14 @@ jobs:
             arch: aarch64
             debug: 1
             extra: " debug"
+          - os: ubuntu-22.04
+            mainmatrix: false
+            arch: android-arm
+            android_arch_abi: armeabi-v7a
+          - os: ubuntu-22.04
+            mainmatrix: false
+            arch: android-aarch64
+            android_arch_abi: arm64-v8a
     steps:
       - uses: actions/checkout@v4
       - name: Setup Ninja
@@ -66,6 +74,15 @@ jobs:
         with:
           version: 1.3.275.0
           cache: true
+      - name: Install Android NDK
+        if: ${{ matrix.arch == 'android-arm' || matrix.arch == 'android-aarch64' }}
+        run: |
+          wget https://dl.google.com/android/repository/android-ndk-r27c-linux.zip -O android-ndk.zip
+          unzip android-ndk.zip -d $HOME
+          export ANDROID_NDK=$HOME/android-ndk-r27c
+          echo "ANDROID_NDK=$ANDROID_NDK" >> $GITHUB_ENV
+          export ANDROID_ARCH_ABI=${{ matrix.android_arch_abi }}
+          echo "ANDROID_ARCH_ABI=$ANDROID_ARCH_ABI" >> $GITHUB_ENV
       - name: Build
         shell: bash
         run: ./presubmit.sh