Fix NPE of A11yManagerService#getAccessibilityShortcutService

Add a null check before flattening the component name.

Bug: 132345003
Test: Null check. Existing unit tests still pass.
Change-Id: I1df04f97d5cad1cecc503b068e3036913aa46811
diff --git a/services/accessibility/java/com/android/server/accessibility/AccessibilityManagerService.java b/services/accessibility/java/com/android/server/accessibility/AccessibilityManagerService.java
index ba4d89c..1f11059 100644
--- a/services/accessibility/java/com/android/server/accessibility/AccessibilityManagerService.java
+++ b/services/accessibility/java/com/android/server/accessibility/AccessibilityManagerService.java
@@ -2205,6 +2205,9 @@
         }
         synchronized(mLock) {
             final UserState userState = getUserStateLocked(mCurrentUserId);
+            if (userState.mServiceToEnableWithShortcut == null) {
+                return null;
+            }
             return userState.mServiceToEnableWithShortcut.flattenToString();
         }
     }