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/

Change-Id: If8032d5c4f4d4d699bed923668fdf1ebc3257bae
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>