[flexiglass] Simplify authentication repository's `checkCredential`.

Currently, we use an async function from `LockPatternChecker` with
callbacks, but suspend the coroutine during the computation. Since this
means we're not using it asynchronously, this CL changes the code to
invoke the synchronous version of this method in `LockPatternUtils`,
which is the same function `LockPatternChecker` calls under the hood.

Bonus: makes the internal convenience function `selectedUserId` a little
easier to use.

Bug: 314757822
Test: Unit tests updated and still pass.
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Change-Id: I459dce0acfb7efbe7a74eb521ffadde9c4f093ad