gradle: Embed assets in baseAssets "Play Asset Delivery" pack

Assets for AABs could be embedded in the application pack but have
significant limitations and size constraints.  Implement the most
simple form of an asset pack that comprises all assets listed in
`manifest.yaml` in an `install-time` pack that is a fixed dependency of
the app.  This makes the files available to `AAssetManager` as soon as
the app is started while being exempt of the 150MB limit.

This matches the existing "asset embedding" functionality provided by
our native APK build.

https://developer.android.com/guide/playcore/asset-delivery/integrate-native

xbuild/src/gradle/mod.rs

@@ -1,6 +1,6 @@
 use crate::cargo::CrateType;
 use crate::{task, BuildEnv, Format, Opt};
-use anyhow::Result;
+use anyhow::{Context, Result};
 use std::path::Path;
 use std::process::Command;
 
@@ -67,6 +67,12 @@ pub fn build(env: &BuildEnv, out: &Path) -> Result<()> {
         dependencies.push_str(&format!("implementation '{}'\n", dep));
     }
 
+    let asset_packs = if config.assets.is_empty() {
+        ""
+    } else {
+        r#"assetPacks = [":baseAssets"]"#
+    };
+
     let app_build_gradle = format!(
         r#"
             plugins {{
@@ -83,6 +89,7 @@ pub fn build(env: &BuildEnv, out: &Path) -> Result<()> {
                     versionCode {version_code}
                     versionName '{version_name}'
                 }}
+                {asset_packs}
             }}
             dependencies {{
                 {dependencies}
@@ -96,6 +103,51 @@ pub fn build(env: &BuildEnv, out: &Path) -> Result<()> {
         dependencies = dependencies,
     );
 
+    let pack_name = "baseAssets";
+    let base_assets = gradle.join(pack_name);
+    // Make sure that any possibly-obsolete asset pack does not clobber the build
+    let _ = std::fs::remove_dir_all(&base_assets);
+
+    if !config.assets.is_empty() {
+        std::fs::create_dir_all(&base_assets)?;
+        let assets = format!(
+            r#"
+            plugins {{
+                id 'com.android.asset-pack'
+            }}
+            assetPack {{
+                packName = "{pack_name}" // Directory name for the asset pack
+                dynamicDelivery {{
+                    // Use install-time to make assets available to AAssetManager
+                    // https://developer.android.com/guide/playcore/asset-delivery/integrate-native
+                    deliveryType = "install-time"
+                }}
+            }}
+            "#,
+        );
+
+        std::fs::write(base_assets.join("build.gradle"), assets)?;
+
+        let target_dir = base_assets.join("src/main/assets");
+        let _ = std::fs::remove_dir_all(&target_dir);
+        std::fs::create_dir_all(&target_dir)?;
+        for asset in &config.assets {
+            let path = env.cargo().package_root().join(asset.path());
+            let target = target_dir.join(asset.path().file_name().unwrap());
+
+            if !asset.optional() || path.exists() {
+                // Make this file or directory available to the `gradle` build system
+                xcommon::symlink(&path, &target).with_context(|| {
+                    format!(
+                        "Failed to make asset file/folder `{}` available to gradle at `{}`",
+                        path.display(),
+                        target.display()
+                    )
+                })?;
+            }
+        }
+    }
+
     if let Some(icon_path) = env.icon.as_ref() {
         let mut scaler = xcommon::Scaler::open(icon_path)?;
         scaler.optimize();

xbuild/src/gradle/settings.gradle

@@ -14,3 +14,4 @@ dependencyResolutionManagement {
 }
 
 include ':app'
+include ':baseAssets'