android: Fix use of `WRITE_EXTERNAL_STORAGE` permission.

[jackrzhang]

This is part of #3075.

This error from React-Native comes up on the debug build when I try to download an image on an emulator running API 28:

On an installation from Play Store, the app crashes.

See this document.

(@zulipbot add "area: android" "crash")
(@zulipbot claim)