binder: combine TLS context objects.

This change creates RpcTransportCtxTls as the base class
of the server and client context classes. RpcTransportCtxTls
implements both addTrustedCertificate and getCertificate. This
is so that server and client can verify each other.

Test: binderRpcTest
Bug: 195166979
Bug: 196422181
Change-Id: I9b3b1ffd346bf8a93c2a5b38871295a8bc4c68ce