This initializes the static field {@link AssetManager#sSystem} which is used
by methods who get a global asset manager using {@link AssetManager#getSystem()}.
They will end up using our bridge asset manager.
{@link Bridge} calls this method after setting up a new bridge.
if (!(AssetManager.sSystem instanceof BridgeAssetManager)) {
// Note that AssetManager() creates a system AssetManager and we override it
// with our BridgeAssetManager.
AssetManager.sSystem = new BridgeAssetManager();
AssetManager.sSystem.makeStringBlocks(null);
}
return AssetManager.sSystem;
