am/applets: Replace includes with forward declarations where applicable

Also resolve places where includes should have been provided, but
weren't.

src/core/hle/service/am/applets/applets.h

@@ -4,14 +4,16 @@
 
 #pragma once
 
-#include <functional>
 #include <memory>
 #include <queue>
 #include "common/swap.h"
-#include "core/hle/kernel/event.h"
 
 union ResultCode;
 
+namespace Kernel {
+class Event;
+}
+
 namespace Service::AM {
 
 class IStorage;

src/core/hle/service/am/applets/software_keyboard.h

@@ -4,7 +4,12 @@
 
 #pragma once
 
+#include <array>
+#include <string>
+#include <vector>
+
 #include "common/common_funcs.h"
+#include "common/swap.h"
 #include "core/hle/service/am/am.h"
 #include "core/hle/service/am/applets/applets.h"