[mle] simplify Start(), Stop(), and SendAnnounce() (#6986)

This commit updates `Start()`, `Stop()` and `SendAnnounce()` in `Mle`
to remove the extra parameters from the `public` versions of these
methods (adding a `private` version with extra parameter for use by
`Mle` class itself). It also adds `StartMode`, `StopMode`, and
`AnnounceMode` enumerations to use as parameter type (instead of
`bool`) in these methods which helps with code readability.

src/core/api/thread_api.cpp

@@ -481,11 +481,11 @@ otError otThreadSetEnabled(otInstance *aInstance, bool aEnabled)
 
     if (aEnabled)
     {
-        error = instance.Get<Mle::MleRouter>().Start(/* aAnnounceAttach */ false);
+        error = instance.Get<Mle::MleRouter>().Start();
     }
     else
     {
-        instance.Get<Mle::MleRouter>().Stop(true);
+        instance.Get<Mle::MleRouter>().Stop();
     }
 
     return error;