[dataset] set key rotation time in security policy on random generation (#5877)

This commit updates `Dataset::Info::GenerateRandom()` to set the Key
Rotation Time in Security Policy component on a Dataset using the
default value (from `KeyManager`). This addresses the issue where the
generated Operational Dataset could potentially use zero for Key
Rotation time.

This commit also changes Security Policy Flags to use default value
from `KeyManager`.

src/core/meshcop/dataset.cpp

@@ -67,11 +67,12 @@ otError Dataset::Info::GenerateRandom(Instance &aInstance)
 
     Clear();
 
-    mActiveTimestamp       = 1;
-    mChannel               = preferredChannels.ChooseRandomChannel();
-    mChannelMask           = supportedChannels.GetMask();
-    mSecurityPolicy.mFlags = aInstance.Get<KeyManager>().GetSecurityPolicyFlags();
-    mPanId                 = Mac::GenerateRandomPanId();
+    mActiveTimestamp              = 1;
+    mChannel                      = preferredChannels.ChooseRandomChannel();
+    mChannelMask                  = supportedChannels.GetMask();
+    mSecurityPolicy.mRotationTime = KeyManager::kDefaultKeyRotationTime;
+    mSecurityPolicy.mFlags        = KeyManager::kDefaultSecurityPolicyFlags;
+    mPanId                        = Mac::GenerateRandomPanId();
 
     SuccessOrExit(error = static_cast<MasterKey &>(mMasterKey).GenerateRandom());
     SuccessOrExit(error = static_cast<Pskc &>(mPskc).GenerateRandom());

src/core/thread/key_manager.cpp

@@ -78,7 +78,7 @@ KeyManager::KeyManager(Instance &aInstance)
     , mKeySwitchGuardEnabled(false)
     , mKeyRotationTimer(aInstance, KeyManager::HandleKeyRotationTimer, this)
     , mKekFrameCounter(0)
-    , mSecurityPolicyFlags(0xff)
+    , mSecurityPolicyFlags(kDefaultSecurityPolicyFlags)
     , mIsPskcSet(false)
 {
     mMasterKey = static_cast<const MasterKey &>(kDefaultMasterKey);

src/core/thread/key_manager.hpp

@@ -115,6 +115,16 @@ typedef Mac::Key Kek;
 class KeyManager : public InstanceLocator, private NonCopyable
 {
 public:
+    enum : uint16_t
+    {
+        kDefaultKeyRotationTime = 672, ///< Default Key Rotation Time (in unit of hours).
+    };
+
+    enum : uint8_t
+    {
+        kDefaultSecurityPolicyFlags = 0xff, ///< Default Security Policy Flags.
+    };
+
     /**
      * This constructor initializes the object.
      *
@@ -448,7 +458,6 @@ private:
     enum
     {
         kMinKeyRotationTime        = 1,
-        kDefaultKeyRotationTime    = 672,
         kDefaultKeySwitchGuardTime = 624,
         kOneHourIntervalInMsec     = 3600u * 1000u,
     };