[CRD iOS] Send key events to the session.

This CL enables the iOS client to send key events (keyboard interactions) to
the remote host. This is the first pass for milestone 1, with the goal of a rough
session experience and a follow up CL will come at a later date to polish this.

Review-Url: https://codereview.chromium.org/2868383003
Cr-Commit-Position: refs/heads/master@{#473677}
24 files changed