Fix reference cycle in InputEventReceiver and Sender.

If the receiver or sender was not properly disposed, then
the underlying input channel might be leaked because the
native peer was holding a strong reference to the object.

Switched to using a weak reference.

Also updated Binder to use a new helper created for this purpose.

Change-Id: I19680bf96d0548777bff02aa1d91874d1e8e41da