Switches SessionManagerClient over to the new StorePolicyEx,
RetrievePolicyEx interface for storing policy. This interface can
handle all the different policy types and doesn't suffer the
combinatorial explosion problem we've been seeing recently.

The new interface should be equivalent to the old one with the one
exception of returned error codes, which are always kGetServiceFail for
the new one compared to kSessionDoesNotExist (user policy) and
kInvalidAccount (device local account policy) for the old one.

BUG=chromium:765644
TEST=Trybots, tested policy fetch on Chell Chromebook

Change-Id: I1ad9c000ae6f6175304780374d0065f5693ba0ea
Reviewed-on: https://chromium-review.googlesource.com/725425


Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Lutz Justen <ljusten@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514433}