Android: downloadFile requests storage permission for app-private file paths

### Description

On Android < 30, `FileTransfer.downloadFile()` appears to require public storage permission even when the destination is an app-private `file://` path under the app data directory.

In our Capacitor app, downloading a ~21 MB SQLite metadata snapshot to an app-private path failed almost immediately on Android 9/10 with:

```text
Error during file transfer
```

The same download succeeds when using deprecated `Filesystem.downloadFile()` with `directory: Directory.Data`, which only checks storage permission for public directories.

### Reproduction shape

```ts
const path = `tmp/data.sqlite`
await Filesystem.mkdir({ path: `tmp`, directory: Directory.Data, recursive: true })
const { uri } = await Filesystem.getUri({ path, directory: Directory.Data })

// Fails quickly on Android < 30 due to storage permission path
await FileTransfer.downloadFile({ url, path: uri })

// Works; writes to app-private storage
await Filesystem.downloadFile({
url,
path,
directory: Directory.Data,
recursive: true,
})
```

### Expected behavior

`FileTransfer.downloadFile()` should not require public/external storage permission when the destination is app-private storage, or it should provide an option/API shape equivalent to `Filesystem.downloadFile({ directory: Directory.Data })`.

### Versions

- `@capacitor/core`: `^8.3.1`
- `@capacitor/file-transfer`: `^2.0.4`
- `@capacitor/filesystem`: `^8.1.2`
- Observed on Android 9/10; API 28 repro confirmed.

Start at the FileTransfer.downloadFile() entry point and compare its Android permission handling with Filesystem.downloadFile({ directory: Directory.Data }). Reproduce the app-private download on Android 9 or API 28, then verify that private destinations work without public storage permission while public storage behavior remains correct.

Written by the indexing model from the issue text.