Introduce client cert alias for HostAuth.

Some email servers require client certificates to be presented to
establish an SSL connection. While this certificate will be maintained
by the system key store, we need to store the "alias" of the certificate
stored in that system store.

Wiring up to use the actual alias will be done in future CL's. It is
currently unused.

Change-Id: I8d1290151342daea9ceb0df8a4088405b44faa81
13 files changed