exchange: Prevent policy provision NPE

If the EAS server fails to resolve provision, canProvision will return null,
which will make crash exchange account wizard trying to attach the policy
to VALIDATE_BUNDLE_POLICY_SET. Just return a NO_POLICY.

aosp change-id: https://android-review.googlesource.com/#/c/78695/
JIRA: CYAN-2356
Issue: https://jira.cyanogenmod.org/browse/CYAN-2356

Change-Id: I2ba3cab33fecf5a75e90e8254653556dd00a3738
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>