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: build

Change-Id: I34696f5a917807f24f855af00e3474aed5dd3228
2 files changed