Fix NPE in JmeAndroidSystem when audio renderer is disabled

[Copilot]

JmeAndroidSystem.newContext() called .equals() directly on settings.getAudioRenderer() without a null guard, crashing with a NullPointerException when users called settings.setAudioRenderer(null) to disable audio — a pattern explicitly supported on desktop.

Changes

• newContext(): Added null check at the head of the audio renderer comparison chain; sets audioRendererType = null when audio is disabled
• newAudioRenderer(): Returns null when settings.getAudioRenderer() == null, consistent with JmeDesktopSystem behavior

// Previously crashed here with NPE when audioRenderer is null
settings.setAudioRenderer(null);
app.start(); // now works; audio is simply disabled