Cache Android builds on GitHub Actions (#1922)

Summary:
Builds up a file with checksums of all `build.gradle` files. Deliberately excludes the `gradle.properties` which changes all the time. The cache shouldn't require nuking except in some rare circumstances.

Pull Request resolved: https://github.com/facebook/flipper/pull/1922

Test Plan: From 41 minutes down to 17: https://github.com/passy/flipper-1/actions/runs/561501374

Reviewed By: mweststrate

Differential Revision: D26427242

Pulled By: passy

fbshipit-source-id: ff1d7370d477c9d7b57c62082ec985108a5ba698

.github/workflows/android-sample.yml

@@ -15,6 +15,15 @@ jobs:
         java-version: 1.8
     - name: Install NDK 21
       run: echo "y" | sudo /usr/local/lib/android/sdk/tools/bin/sdkmanager --install "ndk;21.0.6113669" "ndk;20.0.5594570" --sdk_root=${ANDROID_SDK_ROOT}
+    - name: Compute build cache
+      run: ./scripts/checksum-android.sh checksum-android.txt
+    - uses: actions/cache@v2
+      with:
+        path: |
+          ~/.gradle/caches/modules-*
+          ~/.gradle/caches/jars-*
+          ~/.gradle/caches/build-cache-*
+        key: gradle-${{ hashFiles('checksum-android.txt') }}
     - name: Build with Gradle
       run: ./gradlew :sample:assembleDebug :tutorial:assembleDebug
     - name: upload artifact

.github/workflows/publish-android.yml

@@ -28,6 +28,15 @@ jobs:
       run: echo '${{ secrets.GPG_KEY_CONTENTS }}' | base64 -d > /tmp/secring.gpg
     - name: Update gradle.properties
       run: echo -e "signing.secretKeyRingFile=/tmp/secring.gpg\nsigning.keyId=${{ secrets.SIGNING_KEY_ID }}\nsigning.password=${{ secrets.SIGNING_PASSWORD }}" >> gradle.properties
+    - name: Compute build cache
+      run: ./scripts/checksum-android.sh checksum-android.txt
+    - uses: actions/cache@v2
+      with:
+        path: |
+          ~/.gradle/caches/modules-*
+          ~/.gradle/caches/jars-*
+          ~/.gradle/caches/build-cache-*
+        key: gradle-${{ hashFiles('checksum-android.txt') }}
     - name: Build artifacts
       run: ./gradlew :sample:assembleDebug :sample:assembleRelease && ./gradlew :android:assembleRelease
     - name: Upload Archives