Fix PersistentBlobProvider problems with external storage.

If the user has an external SD card set as their default storage
(notably on a Huawei device, like the Huawei Mate 10 Lite), then
PersistentBlobProvider will try to write a file to the SD card, but
would fail to generate a URI for it due to us missing a line in our
FileProvider setup. This just adds that missing entry.

Special thanks to @aleb2000 for digging into this!

Fixes #8144
Fixes #7726

res/xml/file_provider_paths.xml

@@ -3,6 +3,7 @@
     <external-cache-path name="external_cache" path="." />
     <external-files-path name="external_files" path="." />
 
+    <external-path name="external_path" path="." />
     <external-path name="external_pictures" path="Pictures"/>
     <external-path name="external_video" path="Movies"/>
     <external-path name="external_audio" path="Music"/>