fix(android): gate the latency overhaul behind an experimental toggle, default off

The 5dc24a0 low-latency overhaul regressed badly on some phones. Every piece
of it — decoder ranking, per-SoC vendor keys, the async decode loop, pipeline
thread boosts, the ADPF max-performance bias, game-tagged AAudio, DSCP marking,
the Wi-Fi low-latency lock, HDMI ALLM and the forced TV mode switch — now rides
the "Low-latency mode (experimental)" toggle, default OFF. Off restores the
pre-overhaul pipeline byte-for-byte: the sync poll loop, the platform-default
decoder, and the original format keys (standard low-latency + blind Qualcomm
twin + priority=0 + operating-rate=MAX together).

- New pref key (low_latency_mode_experimental): the old key shipped default-ON,
  so any install that ever saved settings persisted true — flipping the default
  under the old key would leave exactly the regressed devices stuck on.
- DSCP is applied at socket creation, so the toggle reaches the transport via
  NativeBridge.nativeSetLowLatencyMode → transport::set_dscp_default, called in
  the connect choke point before nativeConnect; the core DSCP default reverts
  to off everywhere.
- nativeStartAudio(handle, lowLatencyMode) gates AAudio usage=Game.
- VideoDecoders.pickDecoder now skips `.secure` decoder twins and decoders that
  require FEATURE_SecurePlayback: they need a secure surface, and a secure twin
  could out-score its plain sibling (only it advertising FEATURE_LowLatency),
  which black-screens a clear stream.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

clients/android/native/src/session/planes.rs

@@ -233,14 +233,17 @@ pub extern "system" fn Java_io_unom_punktfunk_kit_NativeBridge_nativeSetVideoSta
     })
 }
 
-/// `NativeBridge.nativeStartAudio(handle)` — start the Opus→AAudio playback thread. No-op if already
-/// started or on a `0` handle. Best-effort: a failure leaves video streaming.
+/// `NativeBridge.nativeStartAudio(handle, lowLatencyMode)` — start the Opus→AAudio playback thread.
+/// `lowLatencyMode` (the experimental toggle) tags the stream usage=Game for the HAL's game-audio
+/// routing. No-op if already started or on a `0` handle. Best-effort: a failure leaves video
+/// streaming.
 #[cfg(target_os = "android")]
 #[no_mangle]
 pub extern "system" fn Java_io_unom_punktfunk_kit_NativeBridge_nativeStartAudio(
     _env: JNIEnv,
     _this: JObject,
     handle: jlong,
+    low_latency_mode: jboolean,
 ) {
     if handle == 0 {
         return;
@@ -251,7 +254,7 @@ pub extern "system" fn Java_io_unom_punktfunk_kit_NativeBridge_nativeStartAudio(
     if guard.is_some() {
         return; // already playing
     }
-    match crate::audio::AudioPlayback::start(h.client.clone()) {
+    match crate::audio::AudioPlayback::start(h.client.clone(), low_latency_mode != 0) {
         Some(p) => *guard = Some(p),
         None => log::error!("nativeStartAudio: playback init failed (video unaffected)"),
     }