Stop putting credential confirmation activity to the home task.

Previously when the user tapped "back" on the credential confirmation
screen, the phone would show home screen automatically. Now this is
handled explicitly in WorkLockActivity to prevent from showing credential
confirmation screens in a loop.

Test: manual, making home and back gesture on work challenge screen
Test: manual, starting work challenge in split-screen mode
Test: manual, invoking work challenge via notification/recents/launcher
Bug: 129045018
Change-Id: Iffd14396e3888416b11d2e7dd2408cab683d4506