Android: Fix rotating EmulationActivity after boot fails

Time for yet another new iteration of working around the
"surface destruction during boot" problem...
This time, the strategy is to use a mutex in MainAndroid.cpp.

Source/Core/Core/Core.h

@@ -99,7 +99,6 @@ void UndeclareAsCPUThread();
 
 std::string StopMessage(bool main_thread, std::string_view message);
 
-bool IsBooting();
 bool IsRunning();
 bool IsRunningAndStarted();       // is running and the CPU loop has been entered
 bool IsRunningInCurrentThread();  // this tells us whether we are running in the CPU thread.
@@ -111,7 +110,6 @@ bool WantsDeterminism();
 // [NOT THREADSAFE] For use by Host only
 void SetState(State state);
 State GetState();
-void WaitUntilDoneBooting();
 
 void SaveScreenShot();
 void SaveScreenShot(std::string_view name);