Determine how to handle `getExternalFilesDir` being null, but an existing AnkiDroid Directory is valid

[david-allison]

• fix(startup): show fatal error dialog if 'AnkiDroid Directory' can't be set due to getExternalFilesDir returning null #19509

For example, on a full build, there will be other functionality which will fail, even if the AnkiDroid Directory is valid

[david-allison]

• [BUG] [Legacy Storage update] - getExternalFilesDir unexpectedly returned null #19554

deckPath: /storage/emulated/0/AnkiDroid
ProductFlavor = play