Fix NPE when calling PasswordViewEntry.getWindowInsetsController().hide()

As PasswordViewEntry didn't always attached to KeyguardPasswordView,
check if the view is attached in case NPE happen when getting null
WindowInsetsController

Fix: 192644416
Test: manual test as issue steps
Test: 1) make a security lock
      2) unlocked to go to launcher
      3) turn off screen
      4) check the log and make sure no crash log.
Change-Id: I13897f05576150b2f5e867c43d20efb02e681cef