Restrict OpenSLES to Android via CMake

[Zopolis4]

OpenSLES code is only compiled on android, but instead of checking via CMake we search for OpenSLES on all platforms, and then compile that code if we find it. Just in case a non-android build finds OpenSLES, both of the files are wrapped in #ifdefs. Instead, this PR will only search for and compile the files on Android, thus not generating a false warning that OpenSLES was not found on regular builds.

[phire]

This is not going to degrade correctly if there is an android build that doesn't find OpenSLES.

Also, OpenSL ES is technically a cross-platform API. Do we really want to put it behind an Android gate? A HAVE_OPENSL_ES define might make more sense.

Edit: Or alternatively, mark it as a required library for android. It's been in there since Android 2.3. Unless there is a chance of Android deprecating it at some point?

[phire]

Also, you remove AudioCommon/OpenSLESStream.h from the list of sources, but it's included anyway?

[Pokechu22]

The way the audio backends currently seem to work is that the header is always included, but if the backend isn't supported it would just produce this:

class OpenSLESStream final : public SoundStream
{
};

which combines with these default values:

dolphin/Source/Core/AudioCommon/SoundStream.h

Lines 11 to 25 in 3595c8b

	class SoundStream
	{
	protected:
	std::unique_ptr<Mixer> m_mixer;
	public:
	SoundStream() : m_mixer(new Mixer(48000)) {}
	virtual ~SoundStream() {}
	static bool IsValid() { return false; }
	Mixer* GetMixer() const { return m_mixer.get(); }
	virtual bool Init() { return false; }
	virtual void SetVolume(int) {}
	// Returns true if successful.
	virtual bool SetRunning(bool running) { return false; }
	};

so that IsValid() always returns false. (IsValid always returns true for all subclasses that aren't removed by the preprocessor except for OpenALStream, which is dependent on both Windows being in use and the run-time existence of openal32.dll, which we don't currently ship.)

We should either replace this with something else (preprocessor checks in AudioCommon.cpp for all backends), or leave the preprocessor checks in OpenSLESStream.h/cpp alone and just make the change in CMakeLists.txt (which should remove the false warning).

(A HAVE_OPENSL_ES check would also make more sense than ANDROID, and in fact there's already a #endif // HAVE_OPENSL in OpenSLESStream.h corresponding to the #ifdef ANDROID, but HAVE_OPENSL itself is never defined or checked anywhere.)

[Zopolis4]

I think there are two paths to take with this PR. Either we try to add cross-platform OpenSLES support or we restrict it to just android. Which one would people prefer?

[Zopolis4]

Given that it is OpenSL ES, I think it makes more sense to restrict it to just android, especially when cubeb works so well.