ci emulator: Cache AVD emulator setup to speed up repeated jobs (#202)

Our CI currently builds and runs the emulator three times, once for
every possible APK source.  It is possible to speed up this setup by
only building the image once as per [1] and storing it using GitHub's
cache system.

[1]: https://github.com/ReactiveCircus/android-emulator-runner#usage

.github/workflows/rust.yml

@@ -94,31 +94,66 @@ jobs:
     strategy:
       matrix:
         source_os: [ubuntu-latest, windows-latest, local]
+    env:
+      api-level: 29
+      emulator-options: -no-window -gpu swiftshader_indirect -noaudio -no-boot-anim -camera-back none
+      arch: x86_64
+      # the `googleapis` emulator target is considerably slower on CI.
+      target: default
+      profile: Nexus 6
+
     steps:
     - name: Checkout
       uses: actions/checkout@v2
+
     - uses: actions/download-artifact@v2
       name: Download hello_world APK
       if: ${{ matrix.source_os != 'local' }}
       id: download
       with:
         name: hello_world_${{ matrix.source_os }}_x86_64-linux-android
+
     - name: Install `cargo-apk` and add `x86_64-linux-android` target
       if: ${{ matrix.source_os == 'local' }}
       run: |
         cargo install --path cargo-apk
         rustup target add x86_64-linux-android
+
+    - name: AVD cache
+      uses: actions/cache@v2
+      id: avd-cache
+      with:
+        path: |
+          ~/.android/avd/*
+          ~/.android/adb*
+        # Bump the trailing number when making changes to the emulator setup below
+        key: avd-${{ env.api-level }}-1
+
+    - name: create AVD and generate snapshot for caching
+      if: steps.avd-cache.outputs.cache-hit != 'true'
+      uses: reactivecircus/android-emulator-runner@v2
+      with:
+        api-level: ${{ env.api-level }}
+        arch: ${{ env.arch }}
+        target: ${{ env.target }}
+        profile: ${{ env.profile }}
+        emulator-options: ${{ env.emulator-options }}
+        disable-animations: true
+        force-avd-creation: false
+        script: echo "Generated AVD snapshot for caching."
+
     - name: Start hello_world example
       uses: reactivecircus/android-emulator-runner@v2
       with:
-        api-level: 29
-        arch: x86_64
-        # the `googleapis` emulator target is considerably slower on CI.
-        target: default
-        profile: Nexus 6
-        emulator-options: -no-window -gpu swiftshader_indirect -noaudio -no-boot-anim -camera-back none
+        api-level: ${{ env.api-level }}
+        arch: ${{ env.arch }}
+        target: ${{ env.target }}
+        profile: ${{ env.profile }}
+        emulator-options: -no-snapshot-save ${{ env.emulator-options }}
         disable-animations: true
+        force-avd-creation: false
         script: ./.github/workflows/android_test.sh "${{ steps.download.outputs.download-path }}"
+
     - name: Upload emulator logs
       uses: actions/upload-artifact@v2
       if: ${{ always() }}