TrustManagerService.java: fix SynchronizeOnNonFinalField warning

Synchronizing on non-final fields is not safe: if the field is ever
updated, different threads may end up locking on different objects.

Test: Build
Change-Id: I3ad24e79c762bd8378c6765e68ae48e9eb86147d
diff --git a/services/core/java/com/android/server/trust/TrustManagerService.java b/services/core/java/com/android/server/trust/TrustManagerService.java
index 5bd485c..377b193 100644
--- a/services/core/java/com/android/server/trust/TrustManagerService.java
+++ b/services/core/java/com/android/server/trust/TrustManagerService.java
@@ -131,7 +131,7 @@
 
     // set to true only if user can skip bouncer
     @GuardedBy("mUsersUnlockedByFingerprint")
-    private SparseBooleanArray mUsersUnlockedByFingerprint = new SparseBooleanArray();
+    private final SparseBooleanArray mUsersUnlockedByFingerprint = new SparseBooleanArray();
 
     private final StrongAuthTracker mStrongAuthTracker;