Remove black flash when unlocking work profile

* Remove black theme of WorkLockActivity. The activity
  below will still not be displayed because there is
  a blankView added in onCreate().

Manual testing steps
* Create a work profile and set a work profile lock
* Launch a work profile app
* Lock and unlock the screen
1. * Enter in the work profile credentials
   * Observe no flash is observed
2. * Return from the lock screen
   * Observe the activity below is not displayed

Bug: 149286440
Test: Manual testing
Change-Id: I18dd66067164e19e81e5fcbc46aa0879fe367a68