Ensure setActiveGroup() does the right thing after a HAL crash.

Before this change, it wasn't deterministic to reset the HIDL layer
after a simulated HAL crash.  It looks like it required a second client
to connect before onError() ultimately corrects mCurrentUserId.
This code forces mCurrentUserId to an unknown state, which ensures it gets
reset when we re-connect to the HAL.

Fixes b/124075073

Test: HAL crash resets mCurrentUserId.
Change-Id: Icca2ad2463dedaf0834e61c744a2ddcac15f6356

services/core/java/com/android/server/biometrics/face/FaceService.java

@@ -705,6 +705,8 @@ public class FaceService extends BiometricServiceBase {
     public void serviceDied(long cookie) {
         super.serviceDied(cookie);
         mDaemon = null;
+
+        mCurrentUserId = UserHandle.USER_NULL; // Force updateActiveGroup() to re-evaluate
     }
 
     @Override