policy: Extract data roaming policy from DeviceSettingsProvider.

This moves the code applying the AllowDataRoaming settings from DeviceSettings to the NetworkConfigurationUpdater which is responsible to apply all other device wide network policies.

- As additional code is required in the updater (listen for device setting changes), this also adds the so far missing (because not strictly required) DeviceNetworkConfigurationUpdater.
- For unit tests of the updater, a interface and implementation of the NetworkDeviceHandler had to be separated.
- The actual application of the AllowDataRoaming setting is now handled in chromeos/network/ .

BUG=323537

Review URL: https://codereview.chromium.org/101593004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242528 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242528