libbinder: keep track of multiple connections in RpcServerTrusty

Correctly keep track of each individual RpcConnection made
to RpcServerTrusty and close the correct one on cleanup.
The code previously assumed that each session would only
have one connection, which is not necessarily the case
in practice with multi-threaded Android clients.

Bug: 224644083
Test: Trusty tests
Change-Id: I12d04de803e0c5df5c658cdbaf3dd0d45e10ef20