checkout submodules for github actions

.github/workflows/android.yml

@@ -9,6 +9,8 @@ jobs:
     runs-on: ubuntu-18.04
     steps:
     - uses: actions/checkout@master
+    - name: Checkout submodules
+      uses: textbook/git-checkout-submodule-action@master
     - name: set up JDK 1.8
       uses: actions/setup-java@master
       with: