Clear WIFI namspace on primary user keystore reset.

This patch makes LocksettingsService clear the WIFI namspace when
Keystore is reset for the primary user.

Test: Using Settings, install a WIFI certificate, tap clear credentials,
      and check that the certificate was removed from the stored
      credentials.
Bug: 189601008
Change-Id: I9364c860250653e50fa8c92c11c510c864f3c3d8