feat(android): streaming cache budget and a hardware decoding switch

- Settings > Storage chooses the streaming cache's budget, 1 to 16 GiB,
  and shows what it holds against it. The bridge gains
  set_stream_cache_budget, which the next connection is built with and
  an open one evicts down to at once. Android keeps the choice in its own
  settings, like the desktop keeps it in desktop.json, and tells the
  engine as it is created. Unset stays at the shared 4 GiB default.
- Settings > Playback can turn hardware decoding off for a device whose
  decoder shows green or torn frames. pstr_mpv sets hwdec before each
  loadfile, so it applies from the next episode.

android/app/src/main/cpp/pstr_mpv.cpp

@@ -31,6 +31,8 @@ extern "C" {
 namespace {
 
 constexpr const char *kProtocol = "pstr";
+/// mpv's `hwdec` when hardware decoding is on. Why this value: `Player()`.
+constexpr const char *kHardwareDecoding = "auto-copy-safe";
 
 struct StreamCookie {
     uint64_t handle;
@@ -216,6 +218,7 @@ class Player {
         option("vo", "libmpv");
         option("force-window", "no");
         option("video-timing-offset", "0");
+        // The default; `load` sets it per file from the viewer's setting.
         // `-copy-safe`, not `-safe`. The direct MediaCodec decoders render into
         // an `ANativeWindow` the decoder is handed at init, and this player has
         // none to give: the picture goes through `vo_libmpv` and an EGL pbuffer,
@@ -224,7 +227,7 @@ class Player {
         // `hevc_mediacodec: Both surface and native_window are NULL` before
         // falling back. The copy variants decode in hardware and hand back
         // ordinary frames, which is what this render path can actually use.
-        option("hwdec", "auto-copy-safe");
+        option("hwdec", kHardwareDecoding);
         option("cache", "yes");
         option("cache-secs", "30");
         option("demuxer-readahead-secs", "30");
@@ -294,8 +297,12 @@ class Player {
     }
 
     bool load(uint64_t handle, double start, const std::string &audio,
-              const std::string &subtitle, bool subtitles) {
+              const std::string &subtitle, bool subtitles,
+              bool hardware_decoding) {
         if (!mpv_) return false;
+        // Per file, before `loadfile`: a device whose decoder shows green
+        // frames is fixed from the next episode without restarting the core.
+        set_string("hwdec", hardware_decoding ? kHardwareDecoding : "no");
         // No wait for a render context. It is created with the render thread and
         // needs no window, and a load that cannot get one plays audio anyway —
         // waiting for a surface here is what made background playback fail after
@@ -822,7 +829,7 @@ extern "C" JNIEXPORT void JNICALL Java_io_narl_protonstream_playback_NativeMpvHo
 extern "C" JNIEXPORT void JNICALL Java_io_narl_protonstream_playback_NativeMpvHost_nativeDetachSurface(JNIEnv *, jobject, jlong handle) noexcept {
     c_boundary([=] { if (auto *p = from(handle)) p->detach(); });
 }
-extern "C" JNIEXPORT jboolean JNICALL Java_io_narl_protonstream_playback_NativeMpvHost_nativeLoad(JNIEnv *env, jobject, jlong handle, jlong stream, jdouble start, jstring audio, jstring subtitle, jboolean subtitles) noexcept {
+extern "C" JNIEXPORT jboolean JNICALL Java_io_narl_protonstream_playback_NativeMpvHost_nativeLoad(JNIEnv *env, jobject, jlong handle, jlong stream, jdouble start, jstring audio, jstring subtitle, jboolean subtitles, jboolean hardware_decoding) noexcept {
     return c_boundary<jboolean>(JNI_FALSE, [=] {
         std::string audio_text;
         std::string subtitle_text;
@@ -832,7 +839,8 @@ extern "C" JNIEXPORT jboolean JNICALL Java_io_narl_protonstream_playback_NativeM
         auto *p = from(handle);
         return static_cast<jboolean>(
             p && p->load(static_cast<uint64_t>(stream), start, audio_text,
-                         subtitle_text, subtitles == JNI_TRUE)
+                         subtitle_text, subtitles == JNI_TRUE,
+                         hardware_decoding == JNI_TRUE)
                 ? JNI_TRUE
                 : JNI_FALSE);
     });