[notifier] update changed flags logging to use multiple lines (#3336)

This commit changes `Notifier::LogChangedFlags()` to allow it to
use multiple lines when there are multiple flags.

src/core/common/notifier.cpp

@@ -206,8 +206,9 @@ exit:
 
 void Notifier::LogChangedFlags(otChangedFlags aFlags) const
 {
-    otChangedFlags                 flags   = aFlags;
+    otChangedFlags                 flags    = aFlags;
-    bool                           isFirst = true;
+    bool                           addSpace = false;
+    bool                           didLog   = false;
     String<kFlagsStringBufferSize> string;
 
     for (uint8_t bit = 0; bit < sizeof(otChangedFlags) * CHAR_BIT; bit++)
@@ -216,20 +217,34 @@ void Notifier::LogChangedFlags(otChangedFlags aFlags) const
 
         if (flags & (1 << bit))
         {
-            SuccessOrExit(string.Append("%s%s", isFirst ? "" : " ", FlagToString(1 << bit)));
+            if (string.GetLength() >= kFlagsStringLineLimit)
-            isFirst = false;
+            {
+                otLogInfoCore("Notifier: StateChanged (0x%08x) %s%s ...", aFlags, didLog ? "... " : "[",
+                              string.AsCString());
+                string.Clear();
+                didLog   = true;
+                addSpace = false;
+            }
+
+            string.Append("%s%s", addSpace ? " " : "", FlagToString(1 << bit));
+            addSpace = true;
+
             flags ^= (1 << bit);
         }
     }
 
 exit:
-    otLogInfoCore("Notifier: StateChanged (0x%04x) [%s] ", aFlags, string.AsCString());
+    otLogInfoCore("Notifier: StateChanged (0x%08x) %s%s] ", aFlags, didLog ? "... " : "[", string.AsCString());
 }
 
 const char *Notifier::FlagToString(otChangedFlags aFlag) const
 {
     const char *retval = "(unknown)";
 
+    // To ensure no clipping of flag names in the logs, the returned
+    // strings from this method should have shorter length than
+    // `kMaxFlagNameLength` value.
+
     switch (aFlag)
     {
     case OT_CHANGED_IP6_ADDRESS_ADDED:

src/core/common/notifier.hpp

@@ -187,7 +187,9 @@ private:
     enum
     {
         kMaxExternalHandlers   = OPENTHREAD_CONFIG_MAX_STATECHANGE_HANDLERS,
-        kFlagsStringBufferSize = 128,
+        kFlagsStringLineLimit  = 70, // Character limit to divide the log into multiple lines in `LogChangedFlags()`.
+        kMaxFlagNameLength     = 25, // Max length for string representation of a flag by `FlagToString()`.
+        kFlagsStringBufferSize = kFlagsStringLineLimit + kMaxFlagNameLength,
     };
 
     struct ExternalCallback