[Base] Moved IsUseNexusForGameBarEnabled from emulator_window to system.h

This will remove direct usage of system specific code from emulator_window

src/xenia/base/system_android.cc

@@ -296,4 +296,5 @@ void ShowSimpleMessageBox(SimpleMessageBoxType type, std::string_view message) {
 
 bool SetProcessPriorityClass(const uint32_t priority_class) { return true; }
 
+bool IsUseNexusForGameBarEnabled() { return false; }
 }  // namespace xe