Add GitHub Actions workflow for Android build and tests

Uses Nix flake to set up the development environment, then runs
unit tests and builds the debug APK.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: psanford

.github/workflows/android.yml

@@ -0,0 +1,28 @@
+name: Android
+on: [push, pull_request]
+jobs:
+
+  build:
+    name: Build/Test
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+
+    steps:
+    - name: Check out code
+      uses: actions/checkout@v4
+
+    - name: Install Nix
+      uses: cachix/install-nix-action@ed706ab2cdb9d2f5533fd430f2c531e7340ed650 # v31
+
+    - name: Setup Nix devShell
+      uses: nicknovitski/nix-develop@9be7cfb4b10451d3390a75dc18ad0465bed4932a # v1
+
+    - name: Initialize gomobile
+      run: make init
+
+    - name: Run unit tests
+      working-directory: android
+      run: ./gradlew testDebugUnitTest
+
+    - name: Build debug APK
+      run: make debug