[mac] move key material update to Init() (#12590)

This commit introduces an `Init()` method to the `Mac` class and
moves the invocation of `KeyManager::UpdateKeyMaterial()` from the
`Mac` constructor into this new method. `Mac::Init()` is then called
from `Instance::AfterInit()`, immediately after `KeyManager` is
initialized.

This ensures that `KeyManager` and other OpenThread core components
are fully constructed and properly initialized before attempting to
update the key material. The key material update interacts with
`SubMac` to configure the MAC keys. Performing this operation during
the `Mac` constructor phase can be problematic because the `KeyManager`
(under `OPENTHREAD_CONFIG_PLATFORM_KEY_REFERENCES_ENABLE`) may not be
fully initialized and ready yet.

src/core/instance/instance.cpp

@@ -433,6 +433,7 @@ void Instance::AfterInit(void)
 #if OPENTHREAD_MTD || OPENTHREAD_FTD
 
     Get<KeyManager>().Init();
+    Get<Mac::Mac>().Init();
 
     // Restore datasets and network information
 

src/core/mac/mac.cpp

@@ -115,7 +115,6 @@ Mac::Mac(Instance &aInstance)
 
     SetEnabled(true);
 
-    Get<KeyManager>().UpdateKeyMaterial();
     SetPanId(mPanId);
     SetExtAddress(randomExtAddress);
     SetShortAddress(GetShortAddress());
@@ -126,6 +125,8 @@ Mac::Mac(Instance &aInstance)
     mMode2KeyMaterial.SetFrom(AsCoreType(&sMode2Key));
 }
 
+void Mac::Init(void) { Get<KeyManager>().UpdateKeyMaterial(); }
+
 void Mac::SetEnabled(bool aEnable)
 {
     mEnabled = aEnable;

src/core/mac/mac.hpp

@@ -130,6 +130,14 @@ public:
      */
     ~Mac(void) { ClearMode2Key(); }
 
+    /**
+     * Initializes the `Mac`.
+     *
+     * This method MUST be called after OpenThread `Instance` is fully initialized (from `Instance::AfterInit()`) and
+     * only after `KeyManager` is also fully initialized.
+     */
+    void Init(void);
+
     /**
      * Starts an IEEE 802.15.4 Active Scan.
      *