Config: Change default value for Dual Core from enabled to disabled

Dual Core is primarily known as one of the first troubleshooting steps when emulation or NetPlay misbehave.

I believe(?) the original intent with having it on by default was likely to support users with weaker machines. However, I believe it would be a lot more reasonable for users to manually enable dual core if they really need the performance boost, rather than it is for them to have come ask us in support channel why their cleanly-installed Dolphin is not emulating their games properly.

Instead, this creates a fragile first impression where a clean install of Dolphin is already set up in a way that leads to inexplicable crashes, subtle timing bugs, and NetPlay desyncs.

Note: This changes only applies to the Desktop application, since mobile devices *do* really need the performance boost.

Source/Core/Core/Config/MainSettings.cpp

@@ -47,7 +47,13 @@ const Info<bool> MAIN_DSP_HLE{{System::Main, "Core", "DSPHLE"}, true};
 const Info<int> MAIN_MAX_FALLBACK{{System::Main, "Core", "MaxFallback"}, 100};
 const Info<int> MAIN_TIMING_VARIANCE{{System::Main, "Core", "TimingVariance"}, 40};
 const Info<bool> MAIN_CORRECT_TIME_DRIFT{{System::Main, "Core", "CorrectTimeDrift"}, false};
-const Info<bool> MAIN_CPU_THREAD{{System::Main, "Core", "CPUThread"}, true};
+#if defined(ANDROID)
+// Currently enabled by default on Android because the performance boost is really needed.
+constexpr bool DEFAULT_CPU_THREAD = true;
+#else
+constexpr bool DEFAULT_CPU_THREAD = false;
+#endif
+const Info<bool> MAIN_CPU_THREAD{{System::Main, "Core", "CPUThread"}, DEFAULT_CPU_THREAD};
 const Info<bool> MAIN_SYNC_ON_SKIP_IDLE{{System::Main, "Core", "SyncOnSkipIdle"}, true};
 const Info<std::string> MAIN_DEFAULT_ISO{{System::Main, "Core", "DefaultISO"}, ""};
 const Info<bool> MAIN_ENABLE_CHEATS{{System::Main, "Core", "EnableCheats"}, false};