Add missing final and @GuardedBy

This is a follow up CL to my previous CL [1], where I forgot to add
final and @GuardedBy to InputMethodManagerService#mStartInputMap.

 [1]: I9921b381e02106dbffff5e0b3d13f0a1245ce807
      6db3bfe33d92127d203ec872a0b353585a99f256

Test: Compile
Bug: 35079353
Change-Id: I29897752e16573a27bf700b1d822889789c35292
diff --git a/services/core/java/com/android/server/InputMethodManagerService.java b/services/core/java/com/android/server/InputMethodManagerService.java
index 2aa17c0..b6b3c43 100644
--- a/services/core/java/com/android/server/InputMethodManagerService.java
+++ b/services/core/java/com/android/server/InputMethodManagerService.java
@@ -536,7 +536,8 @@
         }
     }
 
-    private WeakHashMap<IBinder, StartInputInfo> mStartInputMap = new WeakHashMap<>();
+    @GuardedBy("mMethodMap")
+    private final WeakHashMap<IBinder, StartInputInfo> mStartInputMap = new WeakHashMap<>();
 
     class SettingsObserver extends ContentObserver {
         int mUserId;