Add note saying Android Studio comes with a JDK so you don't need to download OpenJDK separately

**Your Godot version:**
3.2.3 stable
**Issue description:**
The documentation says to install OpenJDK 8 which is fine, but I found keytool and jarsigner came with Android Studio by default. For me on Windows I found `jarsigner.exe` and `keytool.exe` at `C:/Program Files/Android/Android Studio/jre/bin/`. I think it might be helpful to add a note saying that if you're going to install/have Android Studio already installed then OpenJDK is unnecessary. I got Godot to export to Android fine without OpenJDK.

**URL to the documentation page (if already existing):**
[https://docs.godotengine.org/en/stable/getting_started/workflow/export/exporting_for_android.html](https://docs.godotengine.org/en/stable/getting_started/workflow/export/exporting_for_android.html)