Cache Gradle Managed Devices emulator snapshot.

It currently takes ~50 seconds on each CI run to boot the emulator
and save a snapshot.

This commit saves the snapshot to a github action cache, to save the
cost of booting on each run.

However, compressing and saving the snapshot to cache is also expensive (~30 seconds)
so we don't want to do this for every run. Ideally there would be a precise
mechanism for determining if the snapshot has changed. For now, just use a cache
key encorporating the date and the hash of the top level build.gradle file, so
the cache is generated every week.

.github/workflows/ci.yml

@@ -33,7 +33,7 @@ jobs:
           distribution: 'zulu'
           java-version: '17'
       - name: 'Cache Bazel files'
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: ~/.cache/bazel
           key: ${{ runner.os }}-${{ env.cache-version }}-bazel-build-${{ github.sha }}
@@ -75,7 +75,7 @@ jobs:
           distribution: 'zulu'
           java-version: '17'
       - name: 'Cache Bazel files'
-        uses: actions/cache@v3
+        uses: actions/cache@v4
         with:
           path: ~/.cache/bazel
           key: ${{ runner.os }}-${{ env.cache-version }}-bazel-test-${{ github.sha }}
@@ -110,9 +110,31 @@ jobs:
         run: |
             echo 'KERNEL=="kvm", GROUP="kvm", MODE="0666", OPTIONS+="static_node=kvm"' | sudo tee /etc/udev/rules.d/99-kvm4all.rules
             sudo udevadm control --reload-rules
-            sudo udevadm trigger --name-match=kvm       
+            sudo udevadm trigger --name-match=kvm
+      - name: Set ANDROID_USER_HOME
+        run: echo "ANDROID_USER_HOME=$HOME/.android" >> $GITHUB_ENV
       - name: 'Cache Gradle files'
         uses: gradle/gradle-build-action@v2
+      # Saving the GMD snapshot to cache is expensive
+      # Theoretically we only need to update the cache if the snapshot is changed,
+      # which should only happen if emulator version or system image has changed.
+      # As a workaround, encorporate the year and week to the cache key, so 
+      # the snapshot cache gets regenerated every week
+      # http://man7.org/linux/man-pages/man1/date.1.html
+      - name: Get Date
+        id: get-date
+        run: |
+          echo "date=$(/bin/date -u "+%Y%U")" >> $GITHUB_OUTPUT
+        shell: bash
+      - name: 'Cache GMD snapshot'
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.android/avd/gradle-managed/*
+            ~/.android/adb*
+          # use combination of date (to cover case where emulator or system image version changed) and 
+          # hash of top-level build.gradle (to cover case where AGP or android API level changed)
+          key: ${{ runner.os }}-${{ env.cache-version }}-gmd-${{ steps.get-date.outputs.date }}-${{ hashFiles('gradle-tests/build.gradle') }}
       - name: 'Download local snapshot for tests'
         uses: actions/download-artifact@v4
         with:
@@ -137,4 +159,3 @@ jobs:
           name: test-reports
           path: gradle-tests/**/build/reports/androidTests/
 
-