Don't link application support into shell client library

We have some application support code stuffed in
services/shell/public/cpp which exports symbols to be
fixed up at runtime by the runner for early process
initialization. Any dependents will in turn export these
symbols, often unnecessarily.

This removes these sources from the public/cpp target
and introduces a new application_support target for
Mojo binaries to link against.

BUG=611911
R=ben@chromium.org

Review-Url: https://codereview.chromium.org/1978083004
Cr-Commit-Position: refs/heads/master@{#393765}