Implemented consumer management unenrollment.

Implemented consumer management unenrollment.

To implement this feature:
  - Added ConsumerUnenrollmentHandler and ConsumerUnenrollmentHandlerFactory.
  - Added DevcieCloudPolicyManagerChromeOS::Unregister() and Disconnect().
  - Added CloudPolicyService::Unregister().
  - Added unenrollment related stages in ConsumerManagementStage.
  - Extended ConsumerManagementNotification to show unenroll notifications.

Also, the device cloud policy initializer is restarted after the unenrollment
so that we can implement reboot-free enrollment in the future. To implement
this feature:
  - Added OnDeviceCloudPolicyManagerDisconnected().
  - Added BrowserPolicyConnectChromeOS::RestartDeviceCloudPolicyInitializer()
  - Fixed the initiailzer so that it uses ConsumerManagementService to check if
    the device is consumer-managed rather using DeviceCloudPolicyStore, which
    doesn't load policies for consumer-managed devices.

BUG=chromium:353050
TEST=unit_tests

Review URL: https://codereview.chromium.org/751703003

Cr-Commit-Position: refs/heads/master@{#311643}