Code cleanup

Replaced usages of AssetManager#get(String) overload with
AssetManager#get(String, Class). Removed net.pjtb.celdroids.OioSession
class. Fixed and re-enabled backface culling for upside down textures in
net.pjtb.celdroids.client.Game class and made all ShapeRenderers draw
filled rectangles upside down.
WifiDirectSelectionScene improvements

WifiDirectModel performs all Android API tasks on the Android UI thread
while ensuring LibGDX tasks are performed on the LibGDX thread. Darken
the background behind WifiDirectSelectionScene like other popup
subscenes. Implemented a rough version of ScrollableListPane to allow a
player to select a Wi-Fi Direct client to connect to rather than
automatically connect to the first one found.