Allow user without a password to clear credentials

Currently the code assumes that the user always has a pin or
password or pattern. If there's no password confirmKeyGuard() will
return false, so the acttivity will finish without doing anything.
With this change if there's no PIN/password/pattern, the credential
clearing task will be launched straightaway as the user presses "OK"
the confirmation prompt.

Bug: 127697771
Test: manual
Change-Id: Iac4af0abfc7430ed197e04f833bf203c3f66f52e