Host: Add a new "UIHasFocus" hook to determine if the UI has focus

We can't use RendererHasFocus for this purpose because of some issues
with exclusive fullscreen, and the new RendererHasFocus implementation
didn't work for non-Render to Main Window cases, since the renderer
window wasn't managed by wx.

Source/Core/DolphinWX/MainAndroid.cpp

@@ -98,6 +98,11 @@ void Host_SetStartupDebuggingParameters()
 {
 }
 
+bool Host_UIHasFocus()
+{
+	return true;
+}
+
 bool Host_RendererHasFocus()
 {
 	return true;