binder: RpcServer / RpcSession add API for certs.

These APIs call into RpcTransportCtx::getCertificate
and RpcTransportClientCtx::addTrustedPeerCertificate,
respectively.

For RpcSession, peer (server) certificates are fixed when
it is constructed.

Test: binderRpcTest
Bug: 195166979
Change-Id: I0d1bd93042895aeb3ab1de4fe6b9d90e73d0d116