mobile_provider: Choose the primary provider in preference to non-primary providers

Look for a 'primary' attribute on a provider to know if it should be
used as the provider when there are multiple providers that match the
same PLMN.

Update unit tests for mobile_provider_lookup_best_match

BUG=chrome-os-partner:3647
TEST=Ensure that APN selection is done properly

(cherry picked from commit 2e44b115b52ace624ee6b687219bb78c3fec9705)

Change-Id: I7624a8333ae27203ead7f01683214a4cd6d79c0f
Reviewed-on: http://gerrit.chromium.org/gerrit/1611
Tested-by: Jason Glasgow <jglasgow@chromium.org>
Reviewed-by: Jason Glasgow <jglasgow@chromium.org>
4 files changed