[BUG] Locale-only {{tts}} selects an unavailable voice instead of an installed voice

### Checked for duplicates?

- [x] This issue is not a duplicate

### Does it also happen in the desktop version?

- [x] This bug does not occur in the latest version of Anki Desktop (26.05)

### What are the steps to reproduce this bug?

1. Use Google as Android's preferred TTS engine, with an installed French voice.
2. Use a locale-only native TTS tag in a card template:

```html
{{tts fr_FR:Front}}
```

3. Review the card.

AnkiDroid reports:

```text
ERROR_NOT_INSTALLED_YET
```

The same failure occurs with `es_ES`, despite installed French and Spanish voices that work in Android's TTS settings.

As a control, pinning an exact installed voice works:

```html
{{tts fr_FR voices=EXACT_INSTALLED_VOICE:Front}}
```

### Expected behaviour

When no voice is explicitly requested, locale-only selection should prefer a matching voice where `unavailable() == false`, and TTS should play normally.

### Actual behaviour

Locale-only selection takes the first matching voice regardless of availability, so it can select a voice marked `KEY_FEATURE_NOT_INSTALLED` and fail with `ERROR_NOT_INSTALLED_YET`.

AnkiDroid detects unavailable voices here:

https://github.com/ankidroid/Anki-Android/blob/ebcf8e0e34921628b9b8a496c66ffd4adbb3705f/AnkiDroid/src/main/java/com/ichi2/anki/TtsVoices.kt#L261-L265

But the locale-only fallback does not check availability here:

https://github.com/ankidroid/Anki-Android/blob/ebcf8e0e34921628b9b8a496c66ffd4adbb3705f/libanki/src/main/java/com/ichi2/anki/libanki/Tts.kt#L95-L101

### Debug info

```text
AnkiDroid Version = 2.24.0 (ebcf8e0e34921628b9b8a496c66ffd4adbb3705f)
Backend Version = 0.1.64-anki25.09.2 (25.09.2 3890e12c9e48c028c3f12aa58cb64bd9f8895e30)
Android Version = 16 (SDK 36)
ProductFlavor = play
Device Info = Google | google | caiman | caiman | Pixel 9 Pro | caiman
WebView Info = [com.google.android.webview | 787104703]: Mozilla/5.0 (Linux; Android 16; Pixel 9 Pro Build/CP1A.260505.005; wv) AppleWebKit/537.36 (KHTML, like Gecko) Version/4.0 Chrome/150.0.7871.47 Mobile Safari/537.36
ACRA UUID = 1c121d51-efa4-4c0c-ad22-8a00d342b296
FSRS = 5.1.0 (Enabled: true)
Crash Reports Enabled = true
```

### Research

- [x] I have checked the manual and FAQ and could not find a solution to this issue
- [ ] I have not tested the latest alpha release