Ensure that LockscreenCredential#close() gets called

Wrap LockscreenCredential objects in try-with-resources.
This ensures the LockscreenCredential's `close()` method gets
called, which clears the `byte[]` field that stores the LSKF.

Bug: 320392352
Test: presubmit

Change-Id: If180967bcfc1d812edd4202b2d049a4b69d6105f
4 files changed