Keep listening after headset/assist triggers when the phone is locked

[yannikism]

Follow-up to #228 / #213: ACTION_VOICE_COMMAND from a Bluetooth headset already reaches Dicio, but it only works while the phone is unlocked. With the phone locked, nothing happens, and after a cold start the first button press is ignored.

What happens today (from logcat)

A long press on the headset button makes the headset send AT+BVRA, and HeadsetService.startVoiceRecognitionByHeadset then starts MainActivity with ACTION_VOICE_COMMAND. This path is the same whether the phone is locked or not.

• Locked: MainActivity receives the intent and starts listening, but it does not show above the keyguard (setShowWhenLocked is only used for the wake word). It is stopped about 130 ms later, and AudioFlinger silences the recording (AF::RecordTrack: setSilenced ... (silenced)), so Vosk only receives silence.
• Cold start: if the process was killed, Android first recreates MainActivity with its original launcher intent. That preloads the Vosk model with tryLoad(null). The assist intent then arrives through onNewIntent, while the model is still Loading(null). tryLoad(listener) returns false in that state, so the press is dropped and a second press is needed.

Changes

1. MainActivity: assist intents (ACTION_ASSIST, ACTION_VOICE_COMMAND) now set setShowWhenLocked(true), as the wake word already does. setTurnScreenOn(true) is used only for ACTION_ASSIST and the wake word. For ACTION_VOICE_COMMAND (headsets) the screen stays off, since the phone is usually in a pocket.
2. New SttForegroundService (foregroundServiceType="microphone"). It is started from MainActivity.onResume() after an assist intent and stops as soon as the STT is no longer listening (at most 60 s). Once the activity is not visible anymore (locked, or the screen turns off while listening), this is what keeps the microphone from being silenced. It is started only while the activity is resumed, because that is when Android 14+ allows starting a microphone foreground service.
3. VoskInputDevice.tryLoad(): if the model is Loading without a listener, the listener is attached, so listening starts once loading finishes.

Tested

Android 16 phone with a Bluetooth headset that starts voice recognition via HFP:

• Unlocked: one press → listens → answers.
• Locked, screen off: one press → the screen stays off, the recording is unsilenced about 300 ms after the press, Vosk recognizes the speech and the answer is spoken.
• Cold start (process killed), locked: one press is enough.
• :app:testDebugUnitTest passes.

Related, not addressed here

• STT service fails when screen is turned off or locked #154 is about the RecognitionService used by other apps (Tasker "Get Voice") with the screen off, which is a different code path. The same foreground-service approach could probably be applied to SttService, but I could not test that.
• Stop playback while listening #204 / Wake word recognition fails during music playback / hard to give additional commands #363 (audio focus / ducking while listening and speaking) are already covered by Initial audio ducking implementation #380, so this PR does not touch audio focus.
• Ability to turn off Wakeword when screen locked. #422 (security of voice commands while locked): this PR makes headset/assist triggers behave like the existing wake word, which already works on the lock screen. If you'd prefer this behind a setting, I'm happy to add one.

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