Use setTaskOverlay to launch ConfirmDeviceCredentialActivity.

WorkLockActivity is launched as a task overlay activity. If it needs to
launch an activity on top of it, the activity should also be a task
overlay.

Bug: 142853039
Test: Follow the issue description
Test: atest WmTests:ActivityOptionsTest
Change-Id: If4cdc892f608df5337223f70333538e33ff593ca