Start udfps enroll after activity animates in

1) Do not request enrollment until activity animation is complete
2) Do not show fingerprint icon until activity animation is complete

Bug: 171353506
Test: manual
Change-Id: I7cdf5fc4888e35c0a7ba38ea622ae9f3fe1a3abf

src/com/android/settings/biometrics/BiometricsEnrollEnrolling.java

@@ -16,6 +16,7 @@
 
 package com.android.settings.biometrics;
 
+import android.annotation.Nullable;
 import android.content.Intent;
 import android.os.UserHandle;
 import android.view.View;
@@ -33,6 +34,7 @@ public abstract class BiometricsEnrollEnrolling extends BiometricEnrollBase
 
     private static final String TAG_SIDECAR = "sidecar";
 
+    @Nullable
     protected BiometricEnrollSidecar mSidecar;
 
     /**