Fix bug where surface was not clipped off during resizing

When dragging the divider in a way such the task size goes through
the following transition

- Half size
- Full screen
- Half size

the surface wasn't clipped off anymore. This was because in full
screen configuration, computeDragResizing() == false thus when
going full screen -> half size, we reset the draw state to
DRAW_PENDING to get notified when it has finished drawn. However,
this also broke clipping.

In order to fix this, we always put the window into a resizing mode
no matter whether the bounds are fullscreen or not.

However, this introduces an ugly flickering on the navigation bar,
when going into docked mode, because the app doesn't draw navigation
bar background in resize mode.

To fix that, we calculate the presence of navigation bar whether the
window is fullscreen, and not just whether it's resizing. For that,
we need to calculate the presence in BackdropFrameRenderer, by using
the insets just sent by window manager.

Change-Id: Idf56df4ae7fefe67d068bc2eeda8dc4d83bbefb7