AppDataDocumentProvider: Add missing `COLUMN_FLAGS` in the default document projectation

Fixes unable to copy files from device to app's internal storage problem

src/pandroid/app/src/main/java/com/panda3ds/pandroid/app/provider/AppDataDocumentProvider.java

@@ -38,6 +38,7 @@ public class AppDataDocumentProvider extends DocumentsProvider {
             Document.COLUMN_DISPLAY_NAME,
             Document.COLUMN_MIME_TYPE,
             Document.COLUMN_LAST_MODIFIED,
+            Document.COLUMN_FLAGS,
             Document.COLUMN_SIZE
     };