android: Reworked audio backends for SDL3 audio API.

This involved moving an `#ifdef` out of SDL_audio.c for thread priority,
so the default ThreadInit now does the usual stuff for non-Android platforms,
the Android platforms provide an implementatin of ThreadInit with their
side of the `#ifdef` and other platforms that implement ThreadInit
incorporated the appropriate code...which is why WASAPI is touched in here.

The Android bits compile, but have not been tested, and there was some
reworkings in the Java bits, so this might need some further fixes still.

src/audio/wasapi/SDL_wasapi_winrt.cpp

@@ -334,6 +334,7 @@ int WASAPI_ActivateDevice(SDL_AudioDevice *device)
 void WASAPI_PlatformThreadInit(SDL_AudioDevice *device)
 {
     // !!! FIXME: set this thread to "Pro Audio" priority.
+    SDL_SetThreadPriority(device->iscapture ? SDL_THREAD_PRIORITY_HIGH : SDL_THREAD_PRIORITY_TIME_CRITICAL);
 }
 
 void WASAPI_PlatformThreadDeinit(SDL_AudioDevice *device)