Kerberos: Remember active account

Stores the active Kerberos account in a user pref variable. Restores
Kerberos credentials on startup. In case there is no KerberosAccounts
policy and the daemon still sleeps, the call to restore credentials also
wakes it up, which might trigger an update of credentials.

BUG=chromium:952244
TEST=Manually tested on device

Change-Id: I08d0fc84646313851db5325ae59c7654416a5a11
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660856
Commit-Queue: Lutz Justen <ljusten@chromium.org>
Reviewed-by: Kush Sinha <sinhak@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672031}
5 files changed