Add support for Play Asset Delivery.

This only adds support for a subset of Play Asset Delivery: this causes a single install-time asset pack to always be present, but doesn't add support for dynamically downloaded asset packs.

platform/android/java/app/build.gradle

@@ -72,6 +72,8 @@ android {
         targetCompatibility versions.javaVersion
     }
 
+    assetPacks = [":assetPacks:installTime"]
+
     defaultConfig {
         // The default ignore pattern for the 'assets' directory includes hidden files and directories which are used by Godot projects.
         aaptOptions {