Preserve synchronization context because OnRetrievedAccountProperties uses it

(This pull request is submitted under MIT/X11)

`OnRetrievedAccountProperties` tries to pass along a synchronization context:

    if (getUsernameAsync != null) {
        getUsernameAsync (accountProperties).ContinueWith (task => {
            // ...
        }, TaskScheduler.FromCurrentSynchronizationContext ());

However, `OnRetrievedAccountProperties` is also called from a continuation inside `OnRedirectPageLoaded` that doesn't preserve the context:

    RequestAccessTokenAsync (code).ContinueWith (task => {
        if (task.IsFaulted) {
            OnError (task.Exception);
        } else {
            // Ouch!
            OnRetrievedAccountProperties (task.Result); 
        }
    });

This pull requests adds `TaskScheduler.FromCurrentSynchronizationContext ()` to this continuation so the code doesn't crash.