android: build improvements

* Use a GitHub action to download the Android NDK
* Use helper functions available on `std.Build` to simplify
  the build script.
* Use various Zig-isms to simplify the code.

FYI, using Nix to seems to be a non-starter as getting any Android
development kits from nixpkgs requires accepting the Android license
agreement and allowing many packages to use unfree licenses. And since
the packages are unfree they are not cached by NixOS so the build
triggers massive memory-hungry builds.

.github/workflows/test.yml

@@ -373,7 +373,6 @@ jobs:
           path: |
             /nix
             /zig
-            ~/Android/ndk
 
       # Install Nix and use that to run our tests so our environment matches exactly.
       - uses: cachix/install-nix-action@2126ae7fc54c9df00dd18f7f18754393182c73cd # v31.9.1
@@ -385,22 +384,20 @@ jobs:
           authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
 
       - name: Setup Android NDK
-        run: |
-          NDK_ROOT="$HOME/Android/ndk"
-          NDK_DIR="$NDK_ROOT/android-ndk-${{ env.ANDROID_NDK_VERSION }}"
-          if [ ! -d "$NDK_DIR" ]; then
-            curl -fsSL -o /tmp/ndk.zip \
-              "https://dl.google.com/android/repository/android-ndk-${{ env.ANDROID_NDK_VERSION }}-linux.zip"
-            mkdir -p $NDK_ROOT
-            unzip -q /tmp/ndk.zip -d $NDK_ROOT
-            rm /tmp/ndk.zip
-          fi
-          echo "ANDROID_NDK_HOME=$NDK_DIR" >> "$GITHUB_ENV"
+        uses: nttld/setup-ndk@ed92fe6cadad69be94a966a7ee3271275e62f779 # v1.6.0
+        id: setup-ndk
+        with:
+          ndk-version: r29
+          add-to-path: false
+          link-to-sdk: false
+          local-cache: true
 
       - name: Build
         run: |
           nix develop -c zig build lib-vt \
             -Dtarget=${{ matrix.target }}
+        env:
+          ANDROID_NDK_HOME: ${{ steps.setup-ndk.outputs.ndk-path }}
 
   build-linux:
     strategy: