Fix race condition in UdfpsKeyguardView init

- Fixes the NPE crash when auth starts/stops repeatedly
- Fixes the UDFPS icon occasionally exploding in size

Bug: 263741015
Bug: 261519205
Test: atest SystemUITests:com.android.systemui.biometrics
Change-Id: I5f82d6bdb1f0d2e6c6660ebe9bf861d14ffcac30