Remove view tree observer

[tobrun]

This PR removes our view tree observer setup on the MapView initialisation code. This concept was used in the past to make sure the view was measured and that calls to getWidth and getHeight returned not 0 values. With the introduction of async rendering however we introduced the following:

  private void onSurfaceCreated() {
    hasSurface = true;
    post(new Runnable() {
      @Override
      public void run() {
        // Initialise only when not destroyed and only once
        if (!destroyed && mapboxMap == null) {
          MapView.this.initialiseMap();
          mapboxMap.onStart();
        }
      }
    });

Which has the same effect as the ViewTreeObserver setup. It will postpone our setup so components as UiSettings and similar can be initialised with a valid height/width. This change unblocks showing a MapView in a RecyclerView when the item itself isn't visible at Activity start (refs #13132).