Updater: Fix exporting updates

* getExternalStorageDirectory was deprecated in API level 29
* Use the proposed Context#getExternalFilesDirs(String) instead

The following message can be found in the logs, followed by a crash:
MediaProvider: Creating a non-default top level directory or
deleting an existing one is not allowed!

Fixes: https://gitlab.com/LineageOS/issues/android/-/issues/3358
Change-Id: I81412985abc0f64d0fccb0d3069ac57ef6b4ffef