Bug 992206: Cleanup fields in |DroidSocketImpl|, r=shuang

|DroidSocketImpl| contains several unused fields that are removed
by this patch.
Bug 992206: Use |SocketConsumerBase| for Bluetooth sockets, r=shuang
Bug 992206: Use |SocketIORunnable| for Bluetooth sockets, r=shuang
Bug 992206: Use |SocketIOEventRunnable| for Bluetooth sockets, r=shuang
Bug 992206: Use |SocketIOReceiveRunnable| for Bluetooth sockets, r=shuang
Bug 992206: Use |SocketIORequestClosingRunnable| for Bluetooth sockets, r=shuang
Bug 992206: Use |SocketIODeleteInstanceRunnable| for Bluetooth sockets, r=shuang
Bug 992206: Use |SocketIOBase| for Bluetooth sockets, r=shuang
Bug 992206: Use |SocketIOTask| for Bluetooth sockets, r=shuang
Bug 992206: Use |SocketIOSendTask| for Bluetooth sockets, r=shuang
Bug 992206: Use |SocketIOShutdownTask| for Bluetooth sockets, r=shuang
Bug 992206: Cleanup interface of |BluetoothSocket|, r=shuang

This patch cleans up the interface of Bluedroid's |BluetoothScoket|
to look more similar the interface of |UnixSocketConsumer|, from
which it descends.