Add policy transparency dialog to 'Remove work profile'

* In New COPE mode, no explanation was being shown to the
  user as to why the work profile cannot be removed.
* This CL adds a policy transparency dialog when the
  'Remove work profile' preference is selected.
* This was done by modifying AccountRestrictionHelper to
  set the preference to disabled by admin if the device is
  in New COPE mode and the base restriction
  DISALLOW_REMOVE_MANAGED_PROFILE is enforced.

Bug: 149391073
Test: Manual testing
Change-Id: Iec0f57b988e1d4fd08bca040abd1bb30b6991507
Merged-In: Iec0f57b988e1d4fd08bca040abd1bb30b6991507
(cherry picked from commit 56e82784)