[rx-ra-tracker] use mIsRunning in stale timer callback (#12076)

`RxRaTracker` can run independently of `RoutingManager`. Its stale timer
callback should check its own running state (`mIsRunning`) instead of
`RoutingManager`'s state.

This commit corrects the logic in `HandleStaleTimer()` to use the local
`mIsRunning` flag.

src/core/border_router/rx_ra_tracker.cpp

@@ -881,7 +881,7 @@ void RxRaTracker::DetermineStaleTimeFor(const RoutePrefix &aPrefix, NextFireTime
 
 void RxRaTracker::HandleStaleTimer(void)
 {
-    VerifyOrExit(Get<RoutingManager>().IsRunning());
+    VerifyOrExit(mIsRunning);
 
     LogInfo("Stale timer expired");
     mRsSender.Start();