[mle] define DetachGraecfuly variables as private (#8044)

This commit changes the definitions (member variables, constants, and
methods) related to graceful detach mechanism to be `private` instead
of `protected` since they are only accessed/used by the `Mle` and not
by its sub-class `MleRouter`.

src/core/thread/mle.cpp

@@ -83,10 +83,7 @@ Mle::Mle(Instance &aInstance)
     , mAttachTimer(aInstance, Mle::HandleAttachTimer)
     , mDelayedResponseTimer(aInstance, Mle::HandleDelayedResponseTimer)
     , mMessageTransmissionTimer(aInstance, Mle::HandleMessageTransmissionTimer)
-    , mDetachGracefullyTimer(aInstance, Mle::HandleDetachGracefullyTimer)
     , mParentLeaderCost(0)
-    , mDetachGracefullyCallback(nullptr)
-    , mDetachGracefullyContext(nullptr)
     , mAttachMode(kAnyPartition)
     , mParentPriority(0)
     , mParentLinkQuality3(0)
@@ -116,6 +113,9 @@ Mle::Mle(Instance &aInstance)
     , mAlternateChannel(0)
     , mAlternatePanId(Mac::kPanIdBroadcast)
     , mAlternateTimestamp(0)
+    , mDetachGracefullyTimer(aInstance, Mle::HandleDetachGracefullyTimer)
+    , mDetachGracefullyCallback(nullptr)
+    , mDetachGracefullyContext(nullptr)
     , mParentResponseCb(nullptr)
     , mParentResponseCbContext(nullptr)
 {

src/core/thread/mle.hpp

@@ -1770,15 +1770,6 @@ protected:
 
 #endif
 
-    /**
-     * This method indicates whether the device is detaching gracefully.
-     *
-     * @retval TRUE  Detaching is in progress.
-     * @retval FALSE Not detaching.
-     *
-     */
-    bool IsDetachingGracefully(void) { return mDetachGracefullyTimer.IsRunning(); }
-
     Ip6::Netif::UnicastAddress mLeaderAloc; ///< Leader anycast locator
 
     LeaderData    mLeaderData;               ///< Last received Leader Data TLV.
@@ -1796,14 +1787,8 @@ protected:
     TimerMilli    mAttachTimer;              ///< The timer for driving the attach process.
     TimerMilli    mDelayedResponseTimer;     ///< The timer to delay MLE responses.
     TimerMilli    mMessageTransmissionTimer; ///< The timer for (re-)sending of MLE messages (e.g. Child Update).
-    TimerMilli    mDetachGracefullyTimer;
     uint8_t       mParentLeaderCost;
 
-    otDetachGracefullyCallback mDetachGracefullyCallback;
-    void *                     mDetachGracefullyContext;
-
-    static constexpr uint32_t kDetachGracefullyTimeout = 1000;
-
 private:
     static constexpr uint8_t kMleHopLimit        = 255;
     static constexpr uint8_t kMleSecurityTagSize = 4; // Security tag size in bytes.
@@ -1829,6 +1814,8 @@ private:
     static constexpr uint8_t kNextAttachCycleTotalParentRequests       = 2;
     static constexpr uint8_t kNextAttachCycleNumParentRequestToRouters = 1;
 
+    static constexpr uint32_t kDetachGracefullyTimeout = 1000;
+
     enum StartMode : uint8_t // Used in `Start()`.
     {
         kNormalAttach,
@@ -1976,6 +1963,7 @@ private:
     void        ReestablishLinkWithNeighbor(Neighbor &aNeighbor);
     static void HandleDetachGracefullyTimer(Timer &aTimer);
     void        HandleDetachGracefullyTimer(void);
+    bool        IsDetachingGracefully(void) { return mDetachGracefullyTimer.IsRunning(); }
     Error       SendChildUpdateRequest(bool aAppendChallenge, uint32_t aTimeout);
 
 #if OPENTHREAD_FTD
@@ -2111,6 +2099,10 @@ private:
     Ip6::Netif::MulticastAddress mLinkLocalAllThreadNodes;
     Ip6::Netif::MulticastAddress mRealmLocalAllThreadNodes;
 
+    TimerMilli                 mDetachGracefullyTimer;
+    otDetachGracefullyCallback mDetachGracefullyCallback;
+    void *                     mDetachGracefullyContext;
+
     otThreadParentResponseCallback mParentResponseCb;
     void *                         mParentResponseCbContext;
 };