[instance] init BackboneRouter::Local before Mle (openthread#9843)

This commit moves the `mBackboneRouterLocal` before `mMleRouter` in
`Instance` class to ensure that it is initialized and its constructor
is called before. This ensures when `Mle()` constructor invokes the
`ApplyNewMeshLocalPrefix()` method on `BackboneRouter::Local`, it
is already initialized.

src/core/instance/instance.cpp

@@ -124,6 +124,9 @@ Instance::Instance(void)
 #endif
 #if OPENTHREAD_CONFIG_SNTP_CLIENT_ENABLE
     , mSntpClient(*this)
+#endif
+#if OPENTHREAD_FTD && OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
+    , mBackboneRouterLocal(*this)
 #endif
     , mActiveDataset(*this)
     , mPendingDataset(*this)
@@ -178,7 +181,6 @@ Instance::Instance(void)
     , mBackboneRouterLeader(*this)
 #endif
 #if OPENTHREAD_FTD && OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
-    , mBackboneRouterLocal(*this)
     , mBackboneRouterManager(*this)
 #endif
 #if OPENTHREAD_CONFIG_MLR_ENABLE || (OPENTHREAD_FTD && OPENTHREAD_CONFIG_TMF_PROXY_MLR_ENABLE)

src/core/instance/instance.hpp

@@ -510,6 +510,10 @@ class Instance : public otInstance, private NonCopyable
     Sntp::Client mSntpClient;
 #endif
 
+#if OPENTHREAD_FTD && OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
+    BackboneRouter::Local mBackboneRouterLocal;
+#endif
+
     MeshCoP::ActiveDatasetManager  mActiveDataset;
     MeshCoP::PendingDatasetManager mPendingDataset;
     MeshCoP::ExtendedPanIdManager  mExtendedPanIdManager;
@@ -578,7 +582,6 @@ class Instance : public otInstance, private NonCopyable
 #endif
 
 #if OPENTHREAD_FTD && OPENTHREAD_CONFIG_BACKBONE_ROUTER_ENABLE
-    BackboneRouter::Local   mBackboneRouterLocal;
     BackboneRouter::Manager mBackboneRouterManager;
 #endif