libbinder: RPC session ID uses the long binder ID

This is 'unguessable' (pending security review and constant time
compare). Right now, it's unclear if we'll go with full TLS for
on-device communication or use some other authentication scheme.
However, this is being used similarly to TLS session tickets.

Bug: 167966510
Test: binderRpcTest
Change-Id: I4c5edd2de6cc3f6ae37b0815e7f45c7a08bac2b1