libbinder: move FdTrigger to RpcSession

We need triggers accessible in RpcSession, RpcServer, and RpcState in
order to shut down the join + connection + rpc handler threads (the
RpcServer connection threads get transformed into becoming RpcSession
threads). In order for this to be used in all these places, the most
convenient location is RpcSession, since the RpcServer header imports
this and the C++ standard doesn't allow for nested classes to be forward
declared.

Bug: 185167543
Test: binderRpcTest
Change-Id: I3e655277bd183fddc6d5001280cf17a10672bea3