CircleCI Android: Use git-crypt

This way we only need to pass one env variable. And the build process is
more similar to ios.

.circleci/config.yml

@@ -20,14 +20,8 @@ jobs:
           name: Analyze
           command: flutter analyze
       - run:
-          name: Decode Android key store
-          command: echo "$KEYSTORE" | base64 -di > android/app/keystore.jks
-      - run:
-          name: Create Google Play key
-          command: echo "$GOOGLE_PLAY_KEY" | base64 -di > android/google-play-api-key.json
-      - run:
-          name: Create Android Key file
-          command: echo "$ANDROID_KEY_FILE" | base64 -di > android/key.properties
+          name: Unlock Secrets
+          command: ./scripts/decrypt_secrets.sh
       - run:
           name: Build
           command: ./scripts/build.sh