[timer] declare Timer constructor as protected (#7213)

This commit updates `Timer` class constructor to be `protected` which
ensures that instances of `Timer` can not be created and only
instances of its sub-classes `TimerMilli` or `TimerMicro` can be
used. This helps avoid potential incorrect use of `Timer` class
directly.

src/core/common/timer.hpp

@@ -84,21 +84,6 @@ public:
      */
     typedef void (&Handler)(Timer &aTimer);
 
-    /**
-     * This constructor creates a timer instance.
-     *
-     * @param[in]  aInstance   A reference to the OpenThread instance.
-     * @param[in]  aHandler    A pointer to a function that is called when the timer expires.
-     *
-     */
-    Timer(Instance &aInstance, Handler aHandler)
-        : InstanceLocator(aInstance)
-        , mHandler(aHandler)
-        , mFireTime()
-        , mNext(this)
-    {
-    }
-
     /**
      * This method returns the fire time of the timer.
      *
@@ -143,6 +128,13 @@ protected:
         LinkedList<Timer> mTimerList;
     };
 
+    Timer(Instance &aInstance, Handler aHandler)
+        : InstanceLocator(aInstance)
+        , mHandler(aHandler)
+        , mNext(this)
+    {
+    }
+
     bool DoesFireBefore(const Timer &aSecondTimer, Time aNow) const;
     void Fired(void) { mHandler(*this); }