Fix SFX and MIDI music not playing on Android

snd_start_digital() opened two separate SDL audio devices - a raw one
for cutscene/audiolog audio and a second via Mix_OpenAudio() for
SFX/MIDI - but Android's audio backends only ever allow one open
non-capture device, so the second open always failed silently.

- Consolidate onto a single Mix_OpenAudio() device; cutscene/audiolog
  PCM is now mixed into its output via Mix_SetPostMix() instead of a
  dedicated device (16-android-audio-single-device.patch)
- Add SDL_GetError() to the failure log so a future open failure is
  actually diagnosable (15-android-audio-error-detail.patch)
- Make NativeMidiInit() report failure on platforms with no real
  implementation (Android included), instead of silently claiming
  success without opening anything (17-android-native-midi-unsupported.patch)
- Strengthen the OpenSL ES driver hint to SDL_HINT_OVERRIDE and log the
  active audio driver as a standing diagnostic (05-android-audio-driver.patch)

android/engine-patches/15-android-audio-error-detail.patch

@@ -0,0 +1,11 @@
+--- a/src/MacSrc/SDLSound.c
++++ b/src/MacSrc/SDLSound.c
+@@ -42,7 +42,7 @@
+     }
+
+     if (Mix_OpenAudio(48000, AUDIO_S16SYS, 2, 2048) < 0) {
+-        ERROR("%s: Couldn't open audio device", __FUNCTION__);
++        ERROR("%s: Couldn't open audio device: %s", __FUNCTION__, SDL_GetError());
+     }
+
+     Mix_AllocateChannels(SND_MAX_SAMPLES);