Make WorkLockActivity theme translucent

* Instead, set the background in the auth_biometric_background view
* This fixes a bug where there is an infinite loop to start the confirm device credential intent

Bug: 234536059
Test: Manual testing with TestDPC

Manual testing steps
* Setup device with TestDPC
* Set a work lock
* Open TestDPC in the work profile
* Select "lock now"
* Verify there is no infinite loop

Change-Id: I866e3d48a66e562050ad89ebc7f2ae0ac0455737