Find and use the right paths for the app database

[PIG208]

When launching the app for the first time, we use LiveGlobalStore._dbFile to find where the sqlite database file should be created at. This varies by platform, and we need to determine a best fit for each.

For iOS, it is suggested to

[u]se ["Library/Application Support"] to store all app data files except those associated with the user’s documents. For example, you might use this directory to store app-created data files, configuration files, templates, or other fixed or modifiable resources that are managed by the app.

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For Android, it is more ambiguous, as I couldn't find an official recommendation. However, there is getDatabasePath intended to be paired with openOrCreateDatabase that we don't use. The path would look like /data/user/0/com.zulip.flutter/databases/. However, path_provider doesn't have a binding for getDatabasePath.

The other options are:

• path_provider's getApplicationSupportDirectory (Flutter's PathUtils.getFilesDir -> getFilesDir -> empirically /data/data/com.zulip.flutter/files/, or /data/user/0/com.zulip.flutter/files from testing)

• path_provider's getApplicationDocumentsDirectory (Flutter's PathUtils.getDataDirectory -> getDir with MODE_PRIVATE).

Neither requires additional permission to be created, and both refers to private storage.

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We might also take the path on Linux in to consideration, while it is not a target platform.

We get "${XDG_DATA_HOME:-~/.local/share}/com.zulip.flutter/" for getApplicationSupportDirectory and ``xdg-user-dir DOCUMENTS -> e.g. ~/Documents for `getApplicationDocumentsDirectory`. The latter is clearly not a fit, and the former is more reasonable.

[gnprice]

The path we already use has been working fine. If we learn a reason it should change, we can open an issue for that then. (Chat thread.)