Use static inner classes in Godot Java code

(cherry picked from commit 23e152040bb728cff2ef06c3f28724dc7e878dbb)

platform/android/java/lib/src/org/godotengine/godot/GodotIO.java

@@ -74,7 +74,7 @@ public class GodotIO {
 
 	public int last_file_id = 1;
 
-	class AssetData {
+	static class AssetData {
 		public boolean eof = false;
 		public String path;
 		public InputStream is;
@@ -233,7 +233,7 @@ public class GodotIO {
 	/// DIRECTORIES
 	/////////////////////////
 
-	class AssetDir {
+	static class AssetDir {
 		public String[] files;
 		public int current;
 		public String path;