Fix work challenge lockscreen being dismissed

Fix the issue of the work challenge lockscreen being dismissed under
the following circumstances:
* The device has a managed (work) profile.
* The work profile has a separate challenge than the primary one.
* The phone is locked, and on the lock screen there's a notification
associated with an activity in the work profile.

In these circumstances, when the user double-taps the notification from
the work profile, they have to enter the primary challenge first and
then the work one.

However, the issue is that the work challenge is shown for a fraction of
a second, after the user enters the primary challenge, and then
dismissed.

The issue is caused by the lock screen still being shown when the
ConfirmDeviceCredentialActivity is started. It is legal to start an
activity when the device is locked, but it will be paused.

The fix is to delay starting the ConfirmDeviceCredentialActivity until
the lock screen is no longer shown.
This is done by listening to the onKeyguardShowingChanged callback from
KeyguardStateController.Callback:
If, in startWorkChallengeIfNecessary, the keyguard is still showing, a
callback listener is added to the KeyguardStateController.
When the callback is invoked the ConfirmDeviceCredentialActivity is
started.

Bug: 141470517
Test: Manual, double-tap on a managed profile notification (see above)
Change-Id: If85a5781cc2054165eb508cd9634980ce228addc