Collapse InetLayerBasis into EndPointBasis

#### Problem

For historical reasons, Inet EndPoints had unnecessarily deep
class hierarchy, originally:

    InetLayerBasis
      EndPointBasis
        TCPEndPoint
        IPEndPointBasis
          UDPEndPoint

Previous PRs project-chip#11135 and project-chip#11145 merged `IPEndPointBasis` with `UDPEndPoint`.

#### Change overview

This change merges `InetLayerBasis` with `EndPointBasis`, so that the
EndPoint class hierarchy is now:

    EndPointBasis
      TCPEndPoint
      UDPEndPoint

#### Testing

CI; no changes to functionality.

src/inet/BUILD.gn

@@ -84,7 +84,6 @@ static_library("inet") {
     "InetInterface.h",
     "InetLayer.cpp",
     "InetLayer.h",
-    "InetLayerBasis.h",
     "InetLayerEvents.h",
     "arpa-inet-compatibility.h",
   ]

src/inet/EndPointBasis.cpp

@@ -34,7 +34,8 @@ namespace Inet {
 
 void EndPointBasis::InitEndPointBasis(InetLayer & aInetLayer, void * aAppState)
 {
-    InitInetLayerBasis(aInetLayer, aAppState);
+    AppState          = aAppState;
+    mInetLayer        = &aInetLayer;
     mLwIPEndPointType = LwIPEndPointType::Unknown;
 }
 
@@ -56,8 +57,9 @@ void EndPointBasis::DeferredFree(System::Object::ReleaseDeferralErrorTactic aTac
 
 void EndPointBasis::InitEndPointBasis(InetLayer & aInetLayer, void * aAppState)
 {
-    InitInetLayerBasis(aInetLayer, aAppState);
-    mSocket = kInvalidSocketFd;
+    AppState   = aAppState;
+    mInetLayer = &aInetLayer;
+    mSocket    = kInvalidSocketFd;
 }
 
 void EndPointBasis::DeferredFree(System::Object::ReleaseDeferralErrorTactic aTactic)
@@ -71,7 +73,8 @@ void EndPointBasis::DeferredFree(System::Object::ReleaseDeferralErrorTactic aTac
 
 void EndPointBasis::InitEndPointBasis(InetLayer & aInetLayer, void * aAppState)
 {
-    InitInetLayerBasis(aInetLayer, aAppState);
+    AppState   = aAppState;
+    mInetLayer = &aInetLayer;
 }
 
 void EndPointBasis::DeferredFree(System::Object::ReleaseDeferralErrorTactic aTactic)

src/inet/EndPointBasis.h

@@ -27,9 +27,8 @@
 #include <inet/InetConfig.h>
 
 #include <inet/IPAddress.h>
-#include <inet/InetLayerBasis.h>
-
 #include <lib/support/DLLUtil.h>
+#include <system/SystemObject.h>
 
 #if CHIP_SYSTEM_CONFIG_USE_SOCKETS
 #include <system/SocketEvents.h>
@@ -47,11 +46,28 @@ struct tcp_pcb;
 namespace chip {
 namespace Inet {
 
+class InetLayer;
+
 /**
  * Basis of internet transport endpoint classes.
  */
-class DLL_EXPORT EndPointBasis : public InetLayerBasis
+class DLL_EXPORT EndPointBasis : public System::Object
 {
+public:
+    /**
+     *  Returns a reference to the Inet layer object that owns this basis object.
+     */
+    InetLayer & Layer() const { return *mInetLayer; }
+
+    /**
+     *  Returns \c true if the basis object was obtained by the specified Inet layer instance.
+     *
+     *  @note
+     *      Does not check whether the object is actually obtained by the system layer instance associated with the Inet layer
+     *      instance. It merely tests whether \c aInetLayer is the Inet layer instance that was provided to \c InitInetLayerBasis.
+     */
+    bool IsCreatedByInetLayer(const InetLayer & aInetLayer) const { return mInetLayer == &aInetLayer; }
+
 protected:
     void InitEndPointBasis(InetLayer & aInetLayer, void * aAppState = nullptr);
     void DeferredFree(System::Object::ReleaseDeferralErrorTactic aTactic);
@@ -90,6 +106,9 @@ class DLL_EXPORT EndPointBasis : public InetLayerBasis
         TCP     = 4
     } mLwIPEndPointType;
 #endif // CHIP_SYSTEM_CONFIG_USE_LWIP
+
+private:
+    InetLayer * mInetLayer; /**< Pointer to the InetLayer object that owns this object. */
 };
 
 } // namespace Inet

src/inet/InetLayer.cpp

@@ -656,7 +656,7 @@ CHIP_ERROR InetLayer::HandleInetLayerEvent(chip::System::Object & aTarget, chip:
     // If the event was droppable, record the fact that it has been dequeued.
     if (IsDroppableEvent(aEventType))
     {
-        InetLayerBasis & lBasis = static_cast<InetLayerBasis &>(aTarget);
+        EndPointBasis & lBasis = static_cast<EndPointBasis &>(aTarget);
         InetLayer & lInetLayer  = lBasis.Layer();
 
         lInetLayer.DroppableEventDequeued();

src/inet/InetLayer.h

@@ -54,7 +54,7 @@
 #include <inet/IPPrefix.h>
 #include <inet/InetError.h>
 #include <inet/InetInterface.h>
-#include <inet/InetLayerBasis.h>
+#include <inet/EndPointBasis.h>
 #include <inet/InetLayerEvents.h>
 
 #if INET_CONFIG_ENABLE_TCP_ENDPOINT

src/inet/UDPEndPoint.h

@@ -242,7 +242,7 @@ class DLL_EXPORT UDPEndPoint : public EndPointBasis
     /**
      * Close the endpoint and recycle its memory.
      *
-     *  Invokes the \c Close method, then invokes the <tt>InetLayerBasis::Release</tt> method to return the object to its
+     *  Invokes the \c Close method, then invokes the <tt>EndPointBasis::Release</tt> method to return the object to its
      *  memory pool.
      *
      *  On LwIP systems, this method must not be called with the LwIP stack lock already acquired.