Fix crash in ConfirmCredentials due to missing fp service.

Fixes bug 27060816

Change-Id: I25fa1ffc65247360f58473f5bc1745abbabb1e33
(cherry picked from commit 0918e5d650119aed5221b8ed3ac378f273216b45)
diff --git a/src/com/android/settings/fingerprint/FingerprintUiHelper.java b/src/com/android/settings/fingerprint/FingerprintUiHelper.java
index cf37050..df09c49 100644
--- a/src/com/android/settings/fingerprint/FingerprintUiHelper.java
+++ b/src/com/android/settings/fingerprint/FingerprintUiHelper.java
@@ -49,7 +49,8 @@
     }
 
     public void startListening() {
-        if (mFingerprintManager.getEnrolledFingerprints(mUserId).size() > 0) {
+        if (mFingerprintManager != null
+                && mFingerprintManager.getEnrolledFingerprints(mUserId).size() > 0) {
             mCancellationSignal = new CancellationSignal();
             mFingerprintManager.setActiveUser(mUserId);
             mFingerprintManager.authenticate(