[Input System] PlayerInput component - ensure any added action map delegates are updated on ActivateInput() (case ISXB-711)

If action maps are added to an asset *after* the asset has been assigned to the PlayerInput component then the delegates need to be updated.

Before this change no On<XXX> callbacks were created for the added maps leading to input loss.

Adding this sync in the ActivateInput() seems like the best compromise.

Author: AlexTyrer

Packages/com.unity.inputsystem/InputSystem/Plugins/PlayerInput/PlayerInput.cs

@@ -950,6 +950,8 @@ public TDevice GetDevice<TDevice>()
         /// </example>
         public void ActivateInput()
         {
+            UpdateDelegates();
+
             m_InputActive = true;
 
             // If we have no current action map but there's a default
@@ -960,6 +962,22 @@ public void ActivateInput()
                 m_CurrentActionMap?.Enable();
         }
 
+        // Users can add and remove actions maps *after* assigning an InputActionAsset to the PlayerInput component.
+        // This ensures "actionTriggered" delegates are assigned for new maps (case isxb-711)
+        //
+        private int m_MapCount = 0;
+        private void UpdateDelegates()
+        {
+            if (m_Actions == null)
+                return;
+            if (m_MapCount != m_Actions.actionMaps.Count)
+            {
+                InstallOnActionTriggeredHook();
+                CacheMessageNames();
+                m_MapCount = m_Actions.actionMaps.Count;
+            }
+        }
+
         /// <summary>
         /// Disable input on the player, by disabling the current action map
         /// </summary>
@@ -1515,6 +1533,8 @@ private void CacheMessageNames()
 
         private void ClearCaches()
         {
+            if (m_ActionMessageNames != null)
+                m_ActionMessageNames.Clear();
         }
 
         /// <summary>