perf(android): low-latency decode overhaul — vendor keys, async loop, system tuning

Close the latency gap on the Android client with per-SoC decoder tuning, an
event-driven decode loop, and full system integration.

- Decoder selection: rank MediaCodecList decoders in Kotlin (hardware/vendor
  preferred, software avoided, FEATURE_LowLatency probed) and create the chosen
  one by name. Per-SoC low-latency keys gated on the codec-name prefix: Qualcomm
  picture-order + low-latency, Exynos (also Google Tensor), Amlogic, HiSilicon;
  MediaTek vdec-lowlatency set unconditionally. operating-rate = MAX (Qualcomm)
  vs priority = 0 (else) are mutually exclusive. NVIDIA/Rockchip/Realtek have no
  vendor key — covered by ranking + the standard low-latency key.

- Async decode loop: AMediaCodec async-notify replaces the poll loop, presenting a
  decoded frame the instant it is ready instead of waiting out a poll interval.
  Behind USE_ASYNC_DECODE with the synchronous loop kept for A/B during bring-up.

- System integration: Wi-Fi FULL_LOW_LATENCY lock and HDMI ALLM
  (setPreferMinimalPostProcessing) for the stream's lifetime; game_mode_config.xml
  opting out of OEM downscaling / FPS overrides.

- Pipeline: boost the data-plane pump + audio thread priorities, AAudio usage=Game,
  DSCP marking on by default on Android, ADPF setPreferPowerEfficiency(false),
  and setFrameRateWithChangeStrategy(ALWAYS) to force the HDMI mode switch on TV.

- lowLatencyMode master toggle (default on) as the escape hatch; the stats HUD now
  shows the resolved decoder name.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/android/native/src/adpf.rs

@@ -28,6 +28,7 @@ type CreateSessionFn = unsafe extern "C" fn(*mut c_void, *const i32, usize, i64)
 type ReportFn = unsafe extern "C" fn(*mut c_void, i64) -> c_int;
 type UpdateTargetFn = unsafe extern "C" fn(*mut c_void, i64) -> c_int;
 type CloseFn = unsafe extern "C" fn(*mut c_void);
+type SetPreferPowerEfficiencyFn = unsafe extern "C" fn(*mut c_void, bool) -> c_int;
 
 /// The entry points we use, resolved once from `libandroid.so`, plus the process-wide manager.
 struct Api {
@@ -35,6 +36,9 @@ struct Api {
     report: ReportFn,
     update_target: UpdateTargetFn,
     close: CloseFn,
+    /// `APerformanceHint_setPreferPowerEfficiency` — NDK **API 35**, so `Option`al even when the
+    /// rest of ADPF resolved (a 33/34 device has the session API but not this one).
+    set_prefer_power_efficiency: Option<SetPreferPowerEfficiencyFn>,
     manager: *mut c_void,
 }
 
@@ -70,11 +74,20 @@ fn resolve_api() -> Option<Api> {
         if manager.is_null() {
             return None;
         }
+        // Optional (API 35): resolve if present, else `None` — the session still works without it.
+        let set_prefer_power_efficiency =
+            libc::dlsym(lib, c"APerformanceHint_setPreferPowerEfficiency".as_ptr());
+        let set_prefer_power_efficiency = (!set_prefer_power_efficiency.is_null()).then(|| {
+            std::mem::transmute::<*mut c_void, SetPreferPowerEfficiencyFn>(
+                set_prefer_power_efficiency,
+            )
+        });
         Some(Api {
             create_session: std::mem::transmute::<*mut c_void, CreateSessionFn>(create_session),
             report: std::mem::transmute::<*mut c_void, ReportFn>(report),
             update_target: std::mem::transmute::<*mut c_void, UpdateTargetFn>(update_target),
             close: std::mem::transmute::<*mut c_void, CloseFn>(close),
+            set_prefer_power_efficiency,
             manager,
         })
     }
@@ -103,6 +116,13 @@ impl HintSession {
         if session.is_null() {
             return None;
         }
+        // Tell the governor NOT to bias this session toward power efficiency (API 35+): our loop is
+        // latency-critical, so we want it kept on fast cores at high clocks over battery savings.
+        // Best-effort; absent below API 35.
+        if let Some(f) = api.set_prefer_power_efficiency {
+            // SAFETY: `session` is the live session just created; the fn takes it + a bool.
+            unsafe { f(session, false) };
+        }
         Some(Self { api, session })
     }