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/assetPacks/installTime/build.gradle

@@ -0,0 +1,8 @@
+apply plugin: 'com.android.asset-pack'
+
+assetPack {
+    packName = "installTime" // Directory name for the asset pack
+    dynamicDelivery {
+        deliveryType = "install-time" // Delivery mode
+    }
+}

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 {

platform/android/java/app/settings.gradle

@@ -1,2 +1,2 @@
-// Empty settings.gradle file to denote this directory as being the root project
-// of the Godot custom build.
+// This is the root directory of the Godot custom build.
+include ':assetPacks:installTime'