Block SAF directory access to /sdcard/Android

This works for target R+ apps, but need to come up with a better story
for target <R. The enforcement in DocumentsUI is target SDK based
and those apps bypass the block_tree flag.

Bug: 157320644
Test: atest DocumentsTest
Change-Id: I93aea7cd5d3b7befea1f78e1cf3f31f8250f1d33

packages/ExternalStorageProvider/src/com/android/externalstorage/ExternalStorageProvider.java

@@ -322,6 +322,11 @@ public class ExternalStorageProvider extends FileSystemProvider {
                 return true;
             }
 
+            if (TextUtils.equals(Environment.DIRECTORY_ANDROID.toLowerCase(),
+                    path.toLowerCase())) {
+                return true;
+            }
+
             return false;
         } catch (IOException e) {
             throw new IllegalArgumentException(